Certainly here in the UK M20 brake shoes are thin on the ground...They made a lot of the bikes but they also scrapped a lot of them and back when worn shoes were relined rather than replaced that activity didn't generate any spare/used pairs...
There are no good patterns that I know of....If the situation is desperate and it's a question of being on the road or not you can fit the later 7" shoes that were fitted to many BSAs. They are readily available...However, they are narrower than the M20 type...I've done that before and they work OK as a temporary fix...Ian
