I am sure a few members know more about this than I do, a couple with Rudge's spring to mind. I have been refurbishing Gt750 Suzuki's for a few decades, and the sprocket inner cover get trashed when a loose dry chain finally lets go, demolishing the sprocket cover, sometimes entering the lower crankcase. Many are repairable but some not. 

One solution: take a good one and have a casting made from it. A small enterprise,  "Mississauga Foundry" 5230 general road, made two for me at a very reasonable price.

Caution there is a shrinkage issue 3/16" per foot, so the holes will be drilled or filed away from the centre accordingly. This is also sand cast so that a fine rough surface has to be dealt with, this take some time, so be prepared.