If your gun only needs lengthening the cones a bit, and the metal is there to do so safely, and you have no plans to sell it, trade it or export it, why not do so? If I understand your proof rules, as posted on this thread, it would not be illegal for you to own and shoot it yourself. You just could not transfer it to anyone else. Fix it where you can enjoy it, shoot it the rest of your life, and let your heirs worry about it being out of proof. Do you not think you could find another gunsmith to do the work? Or is it illegal for them to do so?

SRH

May God bless America and those who defend her.