Likely some minor differences, it still has points IIRC. And you can put any 150 motor, a Rally 180 or 200 or any P motor in any largeframe if you want. The wiring and if you want autolube are solvable issues, I can't really think of more. A P200 motor dropped easily right into my Sprint and Rally 180 motor into a '58 VBA. Whodatschrome will drop a 200 into anything with two wheels.
Same engine casing as a P125? Assuming since it's a rally frame you can put a 150 or 200 engine into it?