I would first contact Matt Parkhouse in Colorado Springs. If you are an MOA member you know Matt from his tech column in the owner’s news magazine. He has historically bought “mystery” transmissions, and opened/rebuilt them during seminars at the national rally or at Airheads tech days. BTW, if you’re not a member of the Airheads club you should consider it.
Cost can vary depending on what needs replaced, and you don’t know until you open it up. There are other options, but likely more expensive than Matt. I once flew out there with a final drive, spent the day with he and Suzanna, repaired the drive, went to dinner with them and flew home.. (full disclosure – I had a legit work purpose that paid for the airfare, but maybe you have some frequent flier miles to use up) Otherwise, shipping shouldn’t be terrible, as an airhead tranny isn’t that big, it’s more the weight.
If anyone knows a good local option chime in, but I trust Matt’s work, and that he would give the fairest price.
Robert (Bobby) Bancroft Vintage Motorworks in Kinsman, OH 330.876.1505
He’s semi-retired but could definitely do the job if he’s interested. People from all over the US ship their vintage bikes to him for repair/restoration. I believe he was a student of Tom Cutter and has much experience working on airheads. Nice guy.