this is one of those "how long is a piece of string" questions.
if you have the money and the skill to do it, anything will fit. although something that big is really a rear job only.
The only engine that fits perfect is the 1.2 Punto 60 engine but that still requires a few little bits and bobs doing (mainly to the exhaust system as it wont fit). other conversions done are the punto sporting 1.2 16V which will fit again but needs alot more work done to the wiring etc. 1.4 Uno Turbo engines have been put in but its a hell of alot of work and expensive and requires major rewelding of the inner arches and reshaping of other bits and pieces.
if you want power, stick in a 1.2 punto 60 engine and turbo it. Tuned, it would be good for 100-120hp