I've done some preliminary measurements and I believe a Z400 engine could fit without too much work, possibly an LTR with some tank and frame modifications.
That help answer your questions?
As far as I can tell, the Z400 engine is going to be just a tad too tall. Plus you would then need to find room for the radiator and the oil tank. But it's hard to tell for certain with the engine still installed in the z400.
What about a RMZ250, DRZ250 or DR350 engine? I haven't been able to get access to any of those engines to measure them.