Wanted. Motorbike style 50cc moped/bike. ( cheap or free)

We're looking for a father/son project to get ready for when my son turns 16, i don't mind him getting a bike but have set the rule that he must be able to maintain it himself, therefore, the best thing to do would be to get a project that i can teach him with. He's nearly 15 but already 6' 1" so really needs a good size race bike or crosser style bike that preferably runs but needs recommissioning/ new tyres, seat etc and has paperwork. I know it's a big ask what with e-bay etc but we are willing to pay a fair price or come to an arrangement for the right bike ( as cheap as possible but preferably free). We are in West Sussex so collection from the south east is no problem. Thanks for reading and please be assured that any offer would be going to a true petrolhead and not someone looking for a quick profit.