When the topic came up on the local BMW forum, nobody had a good local source for even a final drive. If it were mine, I'd be doing it myself, but I can't claim airhead gearbox experience. My first tendency would be to ship it to Guenther Wuest in Indiana. If you want me to tackle it, I can, but it would be a slow process and If I found a surprise, it might still find its way to Guenther.
and if I hadn't taken so long to post, I'd have beaten chip to the Guenther response, but he isn't local.
Anton Largiader and Tom Cutter would also be on my short list for gearbox builders.