Using bajaj parts in PX - help!

Hi everybody,
I Wonder can anyone help advise me -
I have got a PX125 which I use every day.

I have just been given as a gift a used 150cc bajaj engine, complete except for electrics, stator & flywheel, and rear hub. The crank is not healthy but everything else is fine & within tolerance.

I cannot find any serial number on the bajaj engine and I don’t know which model it came from or how old.

I was thinking of rebuilding the engine to keep as a spare in case my px engine breaks down. But before I spend money on new bearings, crank, seals, etc, can anyone advise me on how interchangeable the parts are with vespa parts.
Particularly, :
(1) can I use a vespa px crank in the engine?
(2) if I ever need to use the bajaj engine, will I be able to just bolt on the electrics and flywheel and rear hub from my px?

If I cannot use these parts then I will just sell the bajaj engine instead of spending money on bajaj parts.
Thank you everybody for your help.

Sorry, no experience using a Bajaj engine, its always been piaggio’s. Best bet, is call a Bajaj/ Piaggio dealer with experience of both types.