hey Doc,
I dont know who has told you that you have to prove that you are the rightful owner of the bike, the Corps cant make you prove anything. It's not a street ridin machine so they cant do anything about it. Until the police come to them and say that you have a suspected stolen bike, there is no reason they need that info.
As far as getting a titles. would you happen to know how to get ahold of the guy that owned it before you? A bill of slae might work but you would need who evers name was on the title