Probably needs synchro ring, engagement sleeve, and possibly gearset. Saw a 90 535 with same problem recently. BMW will want to sell you a complete trans. since they don't trust their techs too far; the parts probably will have to "come from Germany" for the same reason, but they are available and the work is not brain surgery. Oh by the way be sure to check the shift fork. We needed to replace the one on the car we serviced; and that was probably the original cause of failure.