I received from Tony all the parts I need to put the heads back together; now I just have to find a few hours to head over to Paul's to put them together! Tony also put a new bushing in the fourth gear and sent me a new set of roller bearings for the main shaft.
Unfortunately, he also found some damage to the first/second gear unit. Something, at some point, got caught between the teeth of this gear and its mate, so I'll need to closely inspect the other gear to see if it's damaged, too. The damage doesn't look severe, but it's enough AND the gear is inexpensive enough that it's easier to just replace it.