I had proline shooters in Inglewood remove a broken screw from the butt of a Mauser K98, sucker was in there deep and was tough to remove. They did a flawless job and were very reasonable on price. No idea if they can thread a barrel though.
Even if they can’t do the barrel see if they can do the screw. They charged a quarter of what other gunsmiths quoted to remove my broken screw and had it back to me in under a week