Posted by: Mike L Clark
Peter - first port of call should be Goolds - they may have patterns to make these.
Alternatively you might be able to adapt a modern brake drum. The problem is most suppliers do not show the sizes.
This site does show sizes but you would need to root through a fair number of examples to find what you need as there is no comparative table of sizes.
I had the luck to discover that the brake drums for the Toyota Aygo are almost an exact fit for my 1925 Salmson. I adapted Toyota pickup drums for my Model H, thinking they would be easy to source for futureproofing - wrong - they became unavailable quite soon afterwards.