Now, don't throw rotten eggs when I say this, but,... JB-Weld comes to mind.
Look up"Leroy's Ramblings" site and look for the article titled Ramblings of a Retired Gunsmith. Scroll down to Tip# 41.
He says he has (no rotten eggs, now) "repaired" such molestations by CAREFULLY waxing a new case or cartridge, plugging the bore ahead of the chamber, smearing JB Weld in the chamber and chambering the waxed case and let cure. Afterwards, he used the case, slathered with something to polish out the chamber. Something like that.
I know it sounds like lunacy, or made-up even, but there's some resourceful old fellas out there who have pulled of some spectacular stunts.
I'm not so sure I'd try it MYSELF, but if I knew an old gunsmith I trusted (getting hard to find) and he said he could fix it that way, I'd think about it, at least. Me, I'm lucky to JB-Weld a set of wooden handles to a set of LEE 6C mould handles without making SOME kind of a mess.
He offers another option for "low pressure" rounds, wherein he bored out the old chamber, threaded it, screwed in new metal and rechambered it.
He's retired, but there may be someone out there who could still do this.