Now about the difficulty in getting replacement gears.
The local AGCO dealer found me a new small (pinion, part 72099047) gear for $230. I went ahead and ordered it. It took 2 weeks to get here, I suspect because the local parts man forgot about it for a week. They could not find the larger gear.
I found that Roberts Farm Equipment in Ohio had a salvage Allis 5020 4WD and also a Massey Ferguson 210, which is supposed to be the same tractor. I bought the Massey gears as a set of 2, in nice shape, for $400 delivered. Nice, knowledgeable, hard working folks at Roberts.
It turns out that the Massey gear has a shorter 'snout' than the Allis gear that I need. The Allis parts catalog lists three different gears, but only one with 33 teeth and 23 splines (72099049). So I suspect that the Massey final drives are slightly different. Oddly enough, the king pin housing on my Allis has MF stamped on it - would that stand for Massey Ferguson?
The Massey gear would probably work if I had a spacer machined to fit, but I would rather use the correct gear. So another week's wait for the Allis gear, and then (fingers crossed) the Allis will be ready for winter work. I've already got the snow blower attached.
I now have a spare new pinion gear (part #72099047) that I would be willing to part with at my cost, $230, if anyone needs one. The other alternative is a $50 restock fee from AGCO.