Alternative Engine for XT125%

Hi....

My lad's just turned 16 and is wanting a bike. Limited funds so we are looking for a winter project but have found nothing suitable/affordable (its amazing what a spares/repair bike fetches. However, recently i brought an XT125R in bits with the aim to rebuild...make a few quid and then purchase a suitble bike for him.... The XT152R is in great condition, never been ridden - all we need to do is fit another engine and re-connect all the electrics.... I was then thinking...if all goes well rather than sell it.... is there a suitable 50CC engine that will slot in the frame? Am i barking or is this an option.... i'm not looking to do a major rebuild, just swap out the engine so it will need to be compatable.... is the Kinrod XT50 similar?

Cheers