Bob, I have been tripping over a late 80's K75 transmission in my garage for over a year now. I'm out near East Aurora, so there would be no shipping on it. You can have it for the same $120 that Filmcamera paid for his.
There are a few differences from the 75 to the 100, but you can make it work by swapping a few parts over from your old one. The main difference is the clutch push rod because the K75 has a different clutch from the K100. All the important internal parts are identical between them so it's just a bolt up swap.