If you want to change powerful engine, post this under Tech:Force Induction.
http://zerotohundred.com/newforums/forumdisplay.php?f=60
You will have lot's of wonderful/awful suggestion.
If you dun want performance gain, and would like to stick to original engine, go back to Proton and change back the same spec 1.6 Campro engine.
at the end of the day, it all depends on your budget and taste.