Thanks Steve. Parts for British bikes are readily available. The later BSAs, Triumphs and Norton's have really good support. There are a few places that cater to the BSA unit singles: bsaunitsingles.com for just about any parts new and used and E & V Engineering shopevengineering.com for machine and motor work. E & V did the crank, rod, bore and head on my bike. There are also some specialists in England like ccm-britain.co.uk. These bikes have somewhat of a cult following and there is a nice little industry around improving and keeping them running.
Scott