Overhauling FIAT's own engine aint a very good idea. Firstly, you'll find the parts hard to find. Secondly they would be hell expensive.
I will suggest, you go for N-1 engine- Its a 1500 CC engine and weighs a lot less than FIAT's own engine. The engine along with suspension and gear will cost you around 60,000 Rs.
Between, for how much are you buying this FIAT? I've seen people selling FIAT for 100,000 only -