There is really no difference between an automotive generator and the generator on your tractor. Some are two brush, some are three brush, but a basic Delco is a basic Delco. So if your local rebuilder can do a bang up job on an automotive generator, he can do the same for your generator. Now if he is used to working only on alternators, that"s a different story.
Here is a link to Classic Generator. Kenneth does a bang up job, and also has any parts you might need. I recently rebuilt a Delco generator that I thought I was going to have to scrap because it had a bad armature. Kenneth fixed me up with a new armature for a very reasonable price.