The cylinder porting is different too. You would have to port it out. Transfers are cut lower and raising transfer roofs without screwing them up is always difficult.. but the easy way might be to shim under the cylinder and shave off the top of the cylinder, thus raising both the transfers and the exhaust.. I'd have to look it up and check-see if that alone would do it. I don't own a PA50I so can't check directly. Another easy way might be to swap cylinders .. might be .. depends on a lot of things that need to be carefully measured first..
A stock 11.5mm Keihin carb from a Pa50II should bolt right up but i'm not positive.
Going from memory, the PA50II variator and mainshaft has 2 flats where it slips on. An additional flat would need to be machined on the PA50I shaft. afaik, shaft diameter is the same.
Then substitute a PA50II rear wheel / gearbox and that should do it.. but i get the feeling i'm forgetting something(s).
A performance exhaust would surely help but i wouldnt wanna get stuck with a really loud bike that only goes 25mph. I would see the total transformation as a serious project. Swapping parts seems like the easy way.