Great little bikes back in the day! The major problem that occurs is general neglect by the owners (usually ham fisted teenagers), and routine neglect of things like oil changes. This can be very difficult to deal with, as the cam runs directly in the head, like 400/4, 550/4 and 550/4, but with a roller bearing crank the first thing to fail is the cam and head. Engine parts are very scarce nowadays, even D.S.S struggled to get some parts for one of his museum bikes, Honda just didn't expect them to last, and most of them were scrapped more than 20years ago. You MIGHT be lucky and find an okay head/cam for sale somewhere, but I doubt it. Good luck with it though!