Plenty on Ebay and I guess Mark has them too.

https://www.ebay.co.uk/sch/i.html?_nkw=BSA+M20+mag+pinion+oil+seal&_sacat=0&_from=R40&_trksid=p4432023.m570.l1313

Ron

email (option): ronpier@talk21.com

Thanks Ron
another senior moment for me given how much time I usually spend on Ebay that I didn't look there first!
That's multi-tasking for you!