Hi Mis-T; you're doing a great job! My concern about the missing teeth would be that the pinion drive would "jump"; maybe causing more damage. I have never experienced that! Could that be from washing too many heavy items over a long period of time? Those were casted parts and were tough. I would think about replacing that worm gear; but boy, is that a booger! You have to pound out the rod and adjust the thrust. Uggh ... wonder if the Amish supplier for Cottage Craft Works does that? They have full replacement units, but for a few hundred bucks! Larry's right; watch on eBay for a cheaper PU.
That is correct; that's a rubber plug that is the replacement for a tiny lead screw and washer that Maytag used to finish the seal of the center plate to the PU. Needless to say, probably because of environmental issues, Maytag stopped using lead in/on their appliances (remember, too, Maytag used to MAKE all of their own parts; they had smelting capability). Larry is right, you are to push it through from the top and then lop it off with a razor blade to flush to the center plate. You can buy the replacement rubber; comes in about a 4" strip.
Replacing that water seal is a must, as well. Very easy to do. The agitators that Maytag used prior to the turquoise ones, still do an effective job of keeping the water from the center seal. When pushed down all way over the stop ring, it created a "vacuum" and Maytag had a breather hole near the base of the agitator to make it easier to remove. I cringe when I see videos of people allowing the agitator to pop off the agitator post when washing; that whole area gets exposed to too much water. People just don't realize, I suppose. They didn't know you could easily replace the stop ring to get the agitator to stay on its post!
The turquoise agitator has a built-in cup for the area around the stem; it isn't something you can buy/install separately. Never remove the agitator until all the water has been drained from the tub. Also, you shouldn't spray the stem or center seal to clean it; just the area on the center plate called the "sediment zone". That was one of Maytag's feature! All the dirt/gunk gets trapped down there during washing so you can just flush it down the drain.
althus ... if you tipped your mom's N2L over and had a lot of oil/water run out of the right angle drive, you really don't have a choice but to remove the tub and pop that center plate off the transmission to see how much oil has escaped. At that point a "flush and fill" would be a great idea to get it to last another 50 years! These machines should only be tipped on the front ... where the "Maytag" decal and the agitator start button is.