I would also vote either 582 or 912, leaning toward the 912. I remember somewhere reading that the acquisition and operational cost even out about 800 hours if comparing new to new. This is because the 582 needs more frequent overhaul and consume more fuel.
The reason I say 582 or 912 is because they are proven on this airframe, have installation kits, engine mounts, etc available. The 582 you can have about a 550 lb airplane. I'm hoping to have about a 600 pound 912 powered Kitfox for my project airplane. Might be wishful thinking though. Will have to see.