The difference between the 500/550/650 engine cradle and the 750 cradle is huge. i dont think you can shoehorn a 750 engine into the other frames without serious modifications. I haven't seen it done here yet, but have seen many other amazing things happen to the 550 platform to make it go faster, none of them include a 750 as the cylinder spacing is so wide and spaced out different . So much so that cylinders couldn't center on the lower cc's crankshaft and the studding on the 750 is expanded compared to the lower cc'd engine and lower cases. .. even if it could have worked, the cranks couldn't stand the up grade.
But could you be talking about maybe shoehorning a full 750 into a 500/550...that would be a feat. It would be a real ballbuster with a high probability of failure, but a successful transplant would start a new trend for sure.
I would watch that ,that would take a high skill level, take plenty of pictures.