It might be really nit-picking (check that term out) but I said title rests with "the last legal owner", not "the original owner." They might not be the same.
Yes, I think "capture papers" would be sufficient to prove ownership, at least at one time. And I am sure a collector would be more than willing to work with you. The problem is that once guns go underground, they circulate in a community that is not big on "doing the right thing". In fact, they usually do the wrong thing because they are not honest, decent, folks.
AFAIK, there is no federal stolen guns report or listing. The ATF stolen guns program is only for FFL holders.