As usual, 2 steps forward and one step back with the 129. I was able to easily swap the brake interlock spring lever off the 1650, and also resolved what I perceived as the engine laboring too hard, despite fiddling with the carburetor. Turned out to be the breaker points needed a little tweaking. Loosened the points cover, started the engine, let it warm up and pushed engine speed control all the way up with the PTO running the mower. Pried the points open just a hair, and I heard the sweet sound of the throttle backing off just a bit, and repeated it until I found the sweet spot. Took her out on the lawn and she was more than holding her own, even in the heavy stuff.
Took a step back when the original seat pan started to crack, I tend to be a little rough on seats. I had a $75 Amazon Gift Certificate, so I took a flier to see if I could spend it on a new seat, and found a decent high back replacement seat for about $71, including shipping. Hopefully it will be here next week.