If you go motorcycle engine, go with an RZ350 engine, essentially the same as the banshee motor but with a power valve system and iit maks almost 20 more horse throughout the powerband. You'll need all the elctronics with the engine, and pipe adaptors other than that it bolts right in.
If your lookin to go multi cyl four stroke or 2 stroke your into lots of custom fab and lots of $...not worth the headace for the minimal gains IMO.