I suppose you could shove a CB750 engine in just about anything if you wanted to Don, but why would you bother?
A modern engine of the same capacity will produce twice the BHP with less weight, better reliability and much better ground clearance, (no big arsed alternators poking out the side etc) so why would you remove it to replace it with an old "Dinosaur Four"?
Anyway, an OEM frame can be braced and fitted with modern components to make it handle and stop better than anything built "back in the day", without losing the classic CB750 look. When I finished this build, I was surprised at how many old farts came up to me to compliment the bike and tell me that they had one "Just like it" back in the day............ Cheers, Terry.