from what I remember from messing with a I and a II, but not at the same time:
variator side of crankshaft
reed valve is one window on a I and 2 on a II, and there is a bump cast in the engine that makes swapping hard
I think the rear wheels and gearing are the same, but I'm not sure.
Try to find Fred (of Fred's guide)- he's still the expert he's always been, just crabbier.