I've said it before! To the most part a 1" 276 is a 1" 276. The other numbers just relate to configuration of jets, slide, float bowl etc. Which can all be easily altered to suit an M20. Ron
If the thread for the big union nut at the bottom of the carb body is reasonable and the nut is tight, any leakage will probably be to do with float level or needle valve not seating. Ron
The Amal Carb Company (Burlen Fuel Systems) do make new bodies for these carbs. At the moment they appear to be out of stock but I suggest you call them and see how long re-stock may take. New carbs are always better than old ones!
REgards, Mark
Hitchcocks currently have a range of 276s in stock. I don't know what size and whether flange or spigot but a random sample shows good availability. (I once bought a hideously rusty Triumph Thunderbird off a builder's lorry because it was fitted with a 276 and I needed one for my Golden Flash! £25 well spent and I think I've still got some Triumph bits left....)
REgards, Mark