If you re-weld new mount locations for the 650 motor, sure it will fit in a 750 frame.
I've tried a SOHC 650 motor in a SOHC 750 and also a DOHC 750 frame... closer fit on the DOHC 750, but still no go without work with either of them.
However it would be easier stuffing a 650 motor in a 750 frame than the other way around.
l8r