Hi Steve,
BSA parts book is available on CD, its a bit hard to follow as all models are in the same book, so you have a page of cylinders, wheel hubs, and so on
Is your's sv or ohv?
Jules Dassen has a website devoted to early BSA's,
http://www.thebsa.tk/He was able to supply me with replica rockerbox castings for my 24 350ohv
As far as I know the ohv L29 engine used the sloper type rockerbox, I think there was someone on before looking for one??
Rick Parkington who writes for Classic Bike mag, has a few L models, he has been building several over the last couple of years
1929 would be the first year AMAL carbs, I think so that would make the search easier
The magneto needs to be clockwise rotation with 45mm spindle height
BSA used the same gearbox design for years and years, but they came in different sizes,
John