Interesting.
I ended up putting it together and I used both blue loctite and two flat springs under that bolt that used to back out. These are real flat springs (like those used for automatic draw bars) not "spring washers" that supposedly don't work.
I've been running the gearbox all day today and it seems to be working fine. I was surprised this ball bearings arrangement is actually pretty good in removing most of play and still allowing the sheave to move up and down the shaft.
The last issue is that the actual speeds don't exactly match what is written on the dial. The belt used has 28x9 630 written on it, but it is 26mm wide. I had to add spacing rings to at least get the top of the range to line up with the dial. It isn't off by a lot, but still I would like to get it to line up.
Measuring those sheaves I think the original belt was 30mm wide. Unfortunately those belts are quite pricey so until this one breaks I think I'll keep it.
Also, based on the drawings in the manual I think one of the sheaves should have a fan, which is not there. I have to think about 3d printing something that will act like a fan maybe.