I was thinking of putting a motercycle engine in to my fiat 500 to replace the stock motor would this be hard to do the motorcycle engine is 1200 cc

well whether its hard is down to your abilitys really.your looking at a fair bit of custom manufacturing

Size isn't the issue, its just getting it to drive the wheels. It's been done before, an Alfa/Intergrale specialist garage built a mental 500 kit. There was one at Auto Italia at Brooklands a couple of years ago, when they had the track. It was mental