New to this site, Kawasaki 1979 Z1R Project, help?

Unfortunately he doesn't have any of the paperwork to do with the bike, it was one he sold to a customer and the customer eventually brought it back and he ended up keeping it for himself, He isn't really keen on keeping records, I'm sure he would have if he knew how valuable it would have been, but I asked him recently and he thought it might be worth 5,000 so I'm glad I asked.