Have a look at the thread on our restoration of the MB. We took a jelly bean sprocket and machined the bore out, added a key way and also machined the boss to the right size. I couldn’t find an off the shelf part that had a bore and boss size to suit. I just buy off who ever is offering the best prices at the time. If you can’t and dont have a mate who can machine them up, Sedgwick do sell them, but brace yourself they know how to charge!