As put above for big power, the coupe 20v turbo engine is the one for ease and huge amounts of power. In stock form it has 220bhp meaning in a Bravo you would be looking at a 0-60 of 6 seconds and a genuine gps 150mph.
the 0-60 is only 6 seconds due to front wheel drive and hence wheel spin. once rolling it would trounce a lot of stock Subaru imprezza's etc.
The benefit of this engine as well is it can be tuned to 250bhp for around £50 as a guy on the coupe forum sells cheap tuning chips. To do this engine conversion though you would need to put a fiat coupe 20v turbo on a ramp stand under it, and looking up, everything mechanical you could see will need to be taken off and put on the Bravo.
If you wanted a bit less power then the HGT has the N/A vesion of this engine which puts out 147bhp and then in later vis models 155bhp. However it would be pointless to convert to one of these as you can pick HGT Bravo's up far cheaper than you could ever convert a normal one for.