well if you go into the cento section there are FAQ's and stickys on engine upgrades and turbo charging.
1st thing i would say is its much easier to strart with a sporting as a base.
the uno turbo requires massive shell mods to fit on top of the brake,suspension,fuel......upgrades to go with it.
not to mention the insurance implications.
check out the threads ive mentioned and welcome to the forum
Now the post is legible and un-Chavved, some answers might be forthcoming!
900cc Cinq. Logical upgrade is to find a smashed Sporting with good running gear and swap it all into the 900 shell.
To go bigger, the 1242cc Punto 60 engine with SPI will fit, but you need to start with a Sporting base (or conversion) to begin with. This is because the 900cc Cinq doesn't use the FIRE engine, whereas the Sporting uses the 1108cc version of the FIRE engine. The 1242cc engine is a bigger version of the 1108 engine. I think the only modification required is a spacer for the exhaust manifold to downpipe connection due to the taller block.
To go beyond this, you'll be looking at major work. The Uno turbo engine will fit, but requires extensive modification to get into a Cinq engine bay as Custard Boy mentioned. An easier swap than the turbo would be the 1242cc 16 valve Punto Sporting engine, but this would require lots of work to get the injection system wired up and plumbed in. The Punto 60 uses a single point injection system like the Cinq Sporting so is almost a straight swap, whereas the Punto 75 and Sporting use multipoint fuel injection which is totally different.
Look on the Cinq/ Seicento boards as a lot of people have already done conversions. They'll be the best people to ask for advice.
As custard said, look in the technical stickies - a 899 engine swap is achievable, but by no means that straightforward. Engine mounts, loom, etc all need to be changed/modified. Ask rallycinq about the swap - he's done a 899 to 1108 swap.