Have a barrel blank turned to the diameter needed to use it as a liner - keep the chamber area to be thick enough to chamber it and run it safely as it is - also incorporate the breech end of the blank and half or more of the threads.
Bore out and sleeve the old barrel over this specially turned liner, and make sure the liner threads are times to the couple or so legacy threads of the original barrel "sleeve" so that when it's screwed ba k on the rifle itlands the rib at 12:00

Does that make sense to you?