Here's some pictures.
Drive cup.
Drive plate
Drive shaft
The nut that had come loose. First is the damaged threads.
The threads on the shaft. Only the first one or two are kinda rounded over from the loose nut rolling around in there. I think they could easily be cleaned up if I had the right size die.
I am assuming all these parts should be replaced. It is tempting to simply fix up those threads and put everything back together with a 1/4" bolt instead of the dowel.
If nothing else, at least I was able to clean 43 years of grass, dirt, and oil out of a few nooks and crannies.