In case anyone stumbles across this thread in a later search, I wanted to update the outcome. Labor Day holiday finally gave me a chance to catch up on a couple of projects, including this one. I TIG welded the wear in the adjustment plate, and then ground it smooth and used a file to touch up the hole. I turned a new adjustment screw and bushings on the lathe. Unfortunately, I did not take the time to stop and take pictures.
After I reassembled the head, I was able to adjust the speed so that it is more-or-less correct as per the dial - within 1-2% of the stated speed, e.g., 510 instead of 500 rpm, 57 instead of 60 rpm, and so on. I'm still getting some scraping when I get into the highest speeds - but this is well above the 3000 rpm limit on the high speed; it only affects the low speed (back gear) - I can only get up to about 440 rpm on the dial (which is about 410 rpm actual). If I worked at it, I expect I could identify the location of the scraping and fix it ... but I decided not to bother. As it stands, I have a full range from 57 - 3000 rpm, except only for a gap from 410-500 rpm -- I can live with that. And best of all, with the change of the motor bearings (described below), it is smooth and quiet at all speeds.
As the previous paragraph suggests, I changed the motor bearings while I was at it. Even though my motor is a 2 hp, and the video on the H&W site is for a 1-1/2 hp, it was still very helpful. But there were some details that were not mentioned or fully described in the video; below is how I handled them:
1) Getting the fixed side of the pulley loose - this was well and truly stuck. I jury-rigged my too-small pulley puller with longer arms to reach the pulley - but even with a good bit of pressure, it still wasn't coming loose, and I didn't want to over-do the pressure. So, I removed the pulley puller, and while spinning the motor, applied heat from a hand-held propane torch to the pulley. After it was heated to 300-400° or so, I put the puller back on, and this time it came loose with just a bit of pressure.
2) Removing the bottom bearing from the end bell -- the bottom bearing appears to be a press fit both on the bearing and in the bell, but a relatively light press fit in the latter. A few taps with a hammer, buffered by a wood block, tapped it out.
3) I removed the bearings using my hydraulic press - just used a couple of pieces of 1/4" thick steel under each side. Since the 1/4 plate was not necessarily bearing on the inner race, this would not be a good procedure if you had any intention of re-using the bearings, but why would you be removing them if you intended to re-use them?? And by this point I had already determined that the lower bearing had a distinct roughness to it. (Yes, possibly caused by step 2 above - but I don't think so; it felt like something that had been there a while. And no, I don't know that I can define exactly what that feels like!)
4) Installing the new bearings - I rooted around and found a couple of scrap pieces, one a piece of pipe and one a bushing, of the right dimension to press on the inner race of the bearings. I turned each of them a little bit on the lathe to be sure that they were square and to tweak the OD a bit. But even with these I wanted to minimize the pressure used to seat the bearings. At the same time, I was hesitant to heat the bearings, since they are a sealed design - I wasn't sure if a bath of hot oil would harm the grease inside. (Can you tell that I have relatively minimal experience working with bearings??) So instead, I put the armature in the freezer and left it overnight. (Note: spouses may or may not be understanding when they encounter said armature in the freezer, even though it was wrapped in a clean cloth ...). Then I had only had to do a light press to seat the bearings.
5) Installing the bottom bearing in the bell - A quick test showed that the bottom bearing was indeed a press fit in the bottom bell. However, I did not want to actually press it into the bell, since the pressure would be applied across the balls to the outer race - seemed like a good way to mess up the new bearing! (Again, may be my inexperience showing ... but better safe than sorry.) So instead, I put the armature with the installed bearings back in the freezer overnight. The next day, I heated up the end bell using a hot air gun - probably would have been fine to use a torch, but since this is an aluminum casting, I didn't want to take a chance of overdoing it. The hot air gun was slower, but worked just fine to heat it all evenly. I would guess I stopped at 300-400°. I set the plate on a table over a hole to accept the shaft, and the bearing dropped right in, no pressure needed. I left it setting, wrapped in a clean cloth to absorb any condensation developing on the cold armature, until all had reached room temperature.
The rest of the assembly was painless, and the motor is back to working fine - and better yet, with the new bearing, I have eliminated the noise that I was hearing in the system. It wasn't bad, but it is nice to have it gone!
So my mill is now officially fully functional - no more fixing to be done. I promptly got to work on a small project that had been waiting for the mill, and it performed beautifully.