I nearly bought the remains of one of the original Mad Max bikes at a swap meet only 3 or 4 years ago mate, possibly the only CB750 iin the movie, (but I haven't seen it for 20 years so don't hold me to that) it was a Norton Featherbed frame, with spoked alloy rimmed wheels and a race tank, but it had been modified for a CB750 engine.
I never found out how much it was as the seller had left his wife in charge while he checked out the swap, and she had no idea what he wanted for it. Pity, it would have been a nice piece of memorabilia. Cheers, Terry.