Now that would be fun , worlds fast go-kart comes to mind , I am not really good with mods my self so would need some one with a bit of experience an know how , do u know eny garages that take conversion work on ?
i know someone who would be able to do it for you but chances are it wont be cheap to do
shame they are heavy for a small car, bike engines are an easy conversion. mind you, you could go for twin bike engine