G/Day Happy Hunter.
The thing with the XS 650 engine is it's very tall and fits very few other frames. It can go into a BSA A7/10 frame, an Enfield frame XT 500/550, even a Norton Featherbed but they need to be modified slightly. Now I have an old XS500 engine and it looks nowhere near as tall as an XS 650 engine. I can do some measurements when I get home if you'r really keen on the idea, but I don't have a frame. Some of the running gear was common to both and if u can get it cheap enough that may be of some use. Regards Mate.