I still have the missing part (it was more than half, so it clung on to the bolt). Perhaps it could be brazed back on and re-tapped.
But I'd worry about the strength and the same thing happening again, I'd guess.
We might have to go for a raw steel look on the stays, transitioning back to the chrome.
It's a shame though, because the detailing on the frame is incredible - never gonna find another one like it, esp in 51cm. Great colour, chromed stays, chromed lugs, gold lug lining, beautiful painted badges, and yet so tasteful. (Photo is pre-fitting/tuning, pls do not laugh at the saddle angle. The stem was non-negotiable, in spite of my attempts at negotiating!)
Super interested in recommendations for someone who might quote to re-unite the two pieces or replace the dropout.