Hey, congratulation! Airheads are great, (as well as addictive). I like my Clymers manual, something else that's useful, if you're pretty mechanical, is the illustrated parts book. If you can find one, they're $50ish, I think Capital Cycle lists them. Several hundred pages of part numbers and exploded airhead drawings. You said you're in the Atlanta area? Spend part of tomorrow on a trip to Blue Moon Cycles. You will LOVE it! You'll find it quite the place for old BMW's.
BTW, I think BMW stole our gearbox out of a 1930's tractor.