I'm sorry to hear that you got a bad bore. I have had a Gahendra for almost 30 years, purchased at Interarms in Alexandria Va.
Although the chamber is a bit sloppy, it shoots fairly well.
I have no clue what threads may have been used or whether barrel threads could be reproduced. One possible alternative maybe to convert it to 577 Snider. A monster cartridge, but slightly shorter and the same rim diameter. (sort of a parent cartridge of the 577-450.)
The interior workings of the Gahendra are a wonder of simplicity in themselves. For much more info on the Gahendra check in at the British Militaria website at yuku. You will find a wealth of info on the cartridge as well as the Snider.