Best fix is to have a new barrel put in place. Any threaded plugs aren't going to have the strength to with stand the pressures of a nitro rifle cartridge. I am not sure a liner wouldn't fail as well where it passes over the plugs.

You could probably plug the holes with threaded inserts and open the chamber for a straight cased black powder cartridge. Wouldn't be much different from blackpowder muzzleloader pressures which are routinely built with threaded plugs in the breech area.