These bikes are very nice, check the tank for rust, stock av7 would be the best bet, adding a different engine would defeat the variator, it could be done, but reconfigureing the driveline would be more effort than it is worth. That said a 50v/mobylette is one of the nicest riding mopeds ever built. See my profile page for how they look minted out. (stock). If you want to hack and weld get a tomos or a puch, much easier to add different engines. Motobecanes can be made fast, but it is expensive, and involves these huge unwieldly pipes that in some cases require the removal of the pedals. If you get that bike return it to stock form, maybe porting it. You will be amazed with it. Read everything Rebel Moby posts, he is the Motobecane GOD around here.