Bikemadinspain (I'm guessing that's not your real name),
Welcome here.
I think most major parts suppliers could sell you a spares book (often a photocopy). Here is a link to Basmotor, in Sweden, who lists the book as their part number SP37 for about EURO 19. Unless you speak Swedish, you will want to choose English as the language.
http://www.basmotor.se/eng/index.htmlFollow the "Books" pull-down to "BSA Spares".
Having said this, before you buy the book(s), the year amd model of the bike should be verified. To save you some searching on this (because several members have the model number charts at hand) perhaps just tell us the frame number (usually on the vertical part of the steering headstock) and the engine number, at the top of the left side of the crankcase right where the crankcase meets the barrels.
Good luck. Take a picture of the mess in a box, so we can share the fun as it grows up.
Richard L.