"Why not use the 75 engine?" I hear you cry.

The Punto 75 engine does look a better starting point than the 60 at first glance. It offers more bhp and torque, and it's the same size as the 60 engine, so why not use it? Simple answer is that the cylinder head is designed to take the Punto 75 intake manifold with four fuel injectors as opposed to the Cento's one mounted in the throttle body. The bolt spacing on the head just doesn't match up, so your cento intake manifold and throttle body won't bolt up. You could fit the Punto 75 manifold, but to make it work with multi point injection you would need to have the Punto 75 ecu and engine bay wiring loom mated up to your cento - not too easy as the Punto ecu's sit on the opposite side of the engine bay. Getting the throttle cable matched up again might be tricky too. The 75's ecu and injectors need to be supplied with between 2.5 and 3.5 bar of fuel pressure (depending on year of donor car) as opposed to the 1.0 bar that the Centos run at - something which you will need to address. Bums.

The 75 engine has a slightly hotter cam than standard, allegedly the same as Novitec offer for 1108's as a fast road option. if you're keen you could buy a 75 engine then do a mix n'match - 75 engine block, Cento head and 75 cam.

edit: I have it on good authority from TBSei that you can fit the Punto 75 engine, head and intake manifold to a SPI cento and use the SPI throttle body. Obviously the 75's injectors would need to be left in place but not wired / piped in to avoid leaving large open holes everywhere!

So it's the Punto 60 engine then?

For the easy swap, yes. Look for the engine codes which are dot stamped on the top of the water pump housing. You may need some emery cloth or similar to clean the face up in order to read them - you're looking at the flat horizontal piece on the left front of the engine (looking from the exhaust side) just above the water tube, next to the cam belt cover.

Engine ID codes:

Punto 60, up to 1997: 176.B1.000
Punto 60, from 1997 : 176.B4.000
Punto 75, all years : 176.A8.000
Punto 55 1108cc engine up to 1997: 176.A6.000
Punto 55 from 1997 / all Cento 1108 SPI: 176.B2.000

Go on then, how do I do it?

Easy way or hard way? Depends on the depth of your wallet and your spanner wielding ability. Let's not forget swearing performance too, this is almost as important as having the car in the first place. Inventive language always helps the job go smoothly, and reduces pain and swelling when you seriously batter yourself when the spanner slips. I can testify to this, but would also advise that the profanity should ideally cease if you get to the stage of needing a mother or elderly neighbour to transport you to the local infirmary. The proliferation of vulgarity seems to have an enraging effect on the good samaritan roughly proportional to the calming effect on yourself.

I seem to be digressing.

Easy way: Pay a mechanic. Bank on roughly a full day's labour plus parts.

Hard way: Do it yourself.

I'll take the hard way. What do I do?

First, get yourself an engine. Check local breakers, free papers, ebay, Fiat Forum etc. Check the engine code when you get there to make sure it's the right one. Ideally you want to make sure it's a good'un, so do some maintenance while it's on the garage (or mum's kitchen) floor. It's so much easier to do when it's out of the car than to fit it, then find it's no good, then repair it in situ. Things you could change:

Head gasket, plus check the head and block faces - have it skimmed if needs be.
Crank oil seal, both ends.
Camshaft oil seal, pulley end.
Water pump.
The following aren't optional, but essential in my view:
Clutch, complete. Note that the Punto 55, 60, 75 and Cento 1108 share the same clutch.
Thermostat.
Cam belt.
Cam belt tensioner pulley.
Cam cover gasket.

So, you're ready now. Engine's sitting there teasing you and you're ready to rock n' roll. What else do you need to get?

Oil (Fiat recommends 15w40) pref. part or fully synthetic.
Oil filter.
Antifreeze, 2 litres will give you a 50:50 mix which is fine.
Engine hoist - from around 20quid a day hire.
New spark plugs - everyone will give you their best recommendation, it's your choice.
Roll of masking tape and a pen - invaluable. Mark up everything you remove so you know where it goes when you put it back.

How do I fit it?

Buy a Haynes or Porter manual. The procedure is covered quite well in there.
Potted version:
Drain oil and water - dispose of in an environmentally friendly manner.
Remove exhaust manifold and front pipe/cat section.
Remove intake manifold and throttle body.
Remove gear linkages - unbolt the front one and pop the balljoint of the rear one.
Remove the speedo cable from the gearbox end.
Remove driveshafts from gearbox? Naaaah, lets do it the easy way:
- Jack up the car and support it so the front bumper is approx. as high off the ground as your new engine is tall. Remove the front wheels.
- Undo the lower strut to hub bolts on both sides.
- Remove the inner CV boot securing clips on the big ends.
- Pull back the hubs and disengage the tripods on the shafts from the cv joint outers.
- Bag up the open ends of the joints so they don't get mucky. Carrier bags and tape will do nicely.
- Remove clutch cable, any remaining water hoses and wires connected to the engine. Check for earth leads etc.
- Support the engine with the hoist.
- Undo the mounts and drop the engine and 'box tothe floor, carefully.
- Separate engine and gearbox
- Put gearbox on new engine (Don't get them mixed up, would be a terrible disappointment)
- Put engine and box back in, reconnect / refill everything. Cento starter and alternator will fit just fine.

Any sticking points?

Some tiddly, one a bit bigger.
You need to use the Cento crank position sensor, Cento oil pressure switch complete with long hollow bolt and 90' elbow.

The exhaust won't fit. You need to either have the front pipe cut and extended by 15mm to get it on, or make a spacer to go between the front pipe and manifold. If you do this, you will also need longer bolts or studs fitting to the manifold to connect it all up.
You can either use the flange cut from an old front pipe, or have one made up. If you have access to facilities, it's not difficult to do - use a gasket as a template. Alternatively, an engineering shop may make you one up.

Right, it's in - what now?

Don't go mad straight away. Fill the sump with fresh oil, put water and antifreeze in the cooling system, put the new plugs in. Make sure everything that you've touched is tightened up and reconnected.
Start the engine but don't rev it. If the oil pressure light doesn't go out within a few seconds, stop the engine and check.
If it does start and the oil light is out, let it tick over until warm. Keep checking for leaks etc. You will get some smells and a little smoke from oily handprints burning off manifolds etc., don't worry. Plumes of smoke are a bad sign, as is fire. Do stop if this happens. Bleed the cooling system via the screws.

Look, it's in and ok, can I go now?

Yep, go on then. Go steady for the first couple of miles - ideally do a mile or two 'shakedown run' and return to base. Check for leaks, check out any problems now.

If everything is ok, then go for a drive. The ecu is an adaptive system, within limits it can adjust on the fly to the new engine. It may take as long as 300 miles to fully settle in.

The new engine will feel different. The power curve is shifted down the rev range, there's more to be had at lower revs than before. Cruising around is more relaxed and easier with less gearchanging needed. There really isn't any need to rev it into the red, you won't gain anything and will shorten engine life. Also bear in mind that the Puntos are rev limited to 6500, but the Cinq ecu will let it go to 7000 - so it's not too good an idea to go hitting it.

I have a 1242 Cinq, and have managed 17.8 seconds 1/4 mile against a standard car at 20 seconds. Modified 1108's tend to run 18.5 seconds or more. Might not sound like much, but at 80mph across the finish line, 0.7 seconds is a good few car's lengths! On a private track you understand, I have seen the needle all the way to the end of the speedo.

I also have a 1242cc turbo, bought as an 1108cc Van Aaken turbo car and converted. If you fancy copying this, it'll do the 1/4 mile in mid 16's. Goes like stink, but needs a speedo which reads higher

source: http://www.fiatforum.com/showthread.php?t=22060