It's a really simple gearbox. I guess the hard part is trying to figure out what parts are OK, and what parts are causing problems. For example, the transmission in my 28 was full of water, and looked like a mess. It did turn, though, so I knew it wasn't too bad. Taking it apart and cleaning the parts revealed some wear and a few good size rust pits, but nothing that looked like it would keep it from working. The most wear was on the back end of the input shaft (main drive gear), where the dog teeth meet with the internal teeth on the 2nd gear sliding gear.
I don't know how you find someone close by to work on an antique transmission. I've always just dove in head first, and figured out how to fix things on my own. It usually isn't that hard to figure out how to do it.