Here are some pics of a 1950 rigid b31 hub.....the bare hub in the pics has the drum riveted to the hub which was how it was done originally, the hub spoked in to a wheel has been modified to have a bolted on drum/sprocket, the drum is a bsa one maybe an a65.
The bare hub has a 46 tooth sprocket ring newly welded on, the original 42 tooth was machined off first, I doubt it is possibly to weld on a new 42 tooth ring sprocket as there would be hardly any strength in a ring of 42 teeth. The drum needed skimming after the welding job, as it was slightly distorted by the welding despite the best efforts of my machinest guy.
Hopefully if I ever have to renew a 46 tooth sprocket part of the ring can be left on the drum to prevent it distorting again. That was also why I did not opt for a 44 tooth ring ....which could have been used.... just about. The 46 tooth ring was made from a solid blank sprocket.
It is true new hubs are hard to find! And knackered ones tricky to refurbish. The difficulties with renewing the sprockets might be why rigid b31s are a bit rarer?