The seat is for a Champion dirt track frame, the tank, you'll have to ask Mule about it;But I will bet money it’s an alloy tank; probably early Yamaha 500. Mule doesn’t take any shortcuts. He is one of the few builders; (vary few) that I would buy a bike from were I not able build for myself; And I have, bought parts from him, though no one will ever see them after the motors are finished. He is on the Forum, ask him about the parts; thats the only way you are gonna get an honest answer.