There are a few about, and it's likely to be a lot less trouble than a repair. There's a strong possibility that John has these available, either as just the body or the whole carb.
I'm not so sure, that there are a few about, was talking to John Williams earlier this week & he said he only had one in stock & that undamaged ones were getting harder & harder to get.
A lot of the Amal twin jets seem to be suffering the same malady, these & the transom bracket seem to be becoming a valuable commodity.
I may yet make an adaptor,a stud welded on a plate so i can use a seagull carb.I have put tape around whats left of the thread and screwed it in and it seems to have held it for the time being. Also got it running in a fashion with an electric drill starter.
Thanks again
Steve