I had one of these for a few years, I was a pile of crap in standard form, banding was shocking, it wasn't sharp and had a very limited speed range. I have a couple of Roundshot cameras and you can see the evolution of the cameras when you compare side by side. I think at that stage Seitz was on a steep learning curve and the Roundshot is an improved version of the Alpa. I contacted Karl Heitz about repairing it, he was next to useless and appeared to know less than I did about the camera and I knew nothing. He had parts though, all extremely expensive so I went elsewhere.
I found a guy (who is now out of business) who built custom panoramic cameras who was prepared to take it on, he had all the right answers to the questions I posed and had some good ideas of modifications. He made a new motor base and controller so the camera had speeds from 1/250 down to 1s. The viewfinder auto shut off was deleted and converted to manual operation. The film back was converted to o-ring style drive like the Roundshots use, so there were 2 o-rings pressing against the backside of the film keeping constant pressure on them. This was the one modification that made the most difference. The Alpa relies on the film back to push hard against the film to drive it, there's some rubber wheels in there that do the driving. This pressure causes the banding because the motor is constantly fighting against the film back pressure. With o-rings in there doing the driving the film back pressure (spring loaded) was reduced to less than half original.
The rubber drive rollers, Karl wanted something like $50 each for them, my repairman found replacements in a hardware store for 50 cents each. They turned out to have better grip than the originals so were superior as well as cheaper.
The adjustable film gate was deleted and a fixed slit put in, it was offset slightly to improve something I can't remember. I think it ran with a 3mm slit but this is from memory, I could be wrong.
When all done the camera ran flawlessly and had a full speed range. I sold it later because I had too much gear and it wasn't being used, I kept the Roundshots because I had a pair and it's always good to have a back up and shoot identical cameras.
Good luck on your repairs, all I can say is you should be able to repair it yourself and there is a lot of scope for improvement if you want to go down that track. I can help too much more as I'm relying on memory, I sold it 3 or 4 years ago.