I was wondering if anyone knew if a 650 out of a 80 xs would fit in a 77 xs360 frame? or possibly a 500xs engine in a 360 frame.

Thanks

A tape measure & some careful calculating oughta answer those questions.

Sounds like a cool concept.

unless youve got an unlimited budget and a lot of skills id say nope , or why? xs650's are easy and cheap enough to find, youd be better off finding one and redoing it.