Original gear box went out on my '74 R90/6 about 3 years ago. At the time, the bike had a little over 100K miles on it. Luckily I was only about 2 miles from home, and made it there in 1st gear.
At first, I looked into repairing it myself. It sounded like it could possibly be a broken pawl spring, which is an inexpensive part. But after much thought, and knowledge gleaned from ADVRider, I decided not to. Mainly due to lack of mechanical skills/tools/experience in that area. I got quotes to ship it out to get fixed, and they were all $600+.
Found a '75 Transmission with 60K miles on ebay. Got it for around $275 shipped to me. I installed it, and have had no problems since. Now it was not exactly an easy task to replace the transmission, but if you go slow, and stay patient, you can do it no problem.
I still have the 74 Transmission sitting in my garage waiting for me to repair. Maybe...someday...
1985 BMW R80
1974 BMW R90/6 (RIP)