This is a tough one. It isn't just bumps and depressions, the whole thing is wonky. If I was going to try to fix it I would take auto putty and fill all the depressions I could find, then I would take 60 grit sandpaper and just go at the whole helmet until the surface was pretty much even all over. It'll be a lot of work, but if you're willing to do it I think there is some potential in there for a decent lid.