I was at my scooter dealers shop and he put on a 63mm BBK without having to bore out the case like most need . I think he said it had ether the GY6 B or your engine as they have a thicker block he said . Since you have it apart and if you have the cabbage I would replace the stock crank as the bearings are weak for a BBK or even a stroker crank . Walter has everything you could ever need for a 150 as that is all he sells parts for
http://www.scooterelements.ecrater.c.../engine?&srn=2