Okay, so I replaced the brushes. Turned out a lot easier than I expected, and all is running well now.
As usual with a vintage bike, I fixed one thing and another broke. On practically my very next ride, the left-hand mirror started swinging freely around. Back when I first got the bike, I noted: “Took off the mirrors, and put the left one back on… because the left mirror was attached to the right side of the handlebar on a clamp, and the right mirror was crossthreaded into the mount on the left side. You all know that Yamaha bikes have left-handed right mirrors, but apparently the previous owner didn’t. Gah. The collar nut is missing from the right-hand mirror, so I couldn’t remount it, but after running a bolt through the left-hand mount from the bottom to straighten the threads, that mirror went on fine.” Later I got new mirrors; in fact, I bought new mirrors twice, as the ones I got in July of 18 were kind of crappy, actually. Vibration from the XS made them entirely useless, and they would not stay adjusted. But I had retained the original clutch perch, as the damaged threads didn’t seem to be a problem at first. Now they were. Over the next several rides, the mirror would repeated jump a thread and get loose, requiring gentle, judicious retightening. Something had to change.
I went on ebay and found this: https://www.ebay.com/itm/45-4012-YA...EFT-SIDE-BRACKET-341-82911-00-00/361317819355. So I ordered one, and when it came I pulled the bad one and laid them side-by-side. Sorry I didn’t take a pic, but trust me, the replica is almost exactly the same. The metal is just the least bit thinner where the lever mounts to the perch, but that’s it, and it’s not thin enough to worry me.
Managed to save the left grip, too, so I didn’t have to replace it; I call that a win.
Here’s the cockpit of my XS650 now:
Emgo bars, the cheapest (and yet, in my opinion, nicest) grips on ebay, plus bar end weights, mirrors, master cylinder, and clutch perch all from ebay. Gauges and indicator cover are used, with thanks to bwthor, and a used clutch lever stolen from my TW200. I’d already put shorties on that bike, and when I got the master cylinder with a shiny metal lever, I felt like I needed a shiny metal lever on the clutch as well. The only thing original there are the switches, and the plastic tiedowns for the wiring that I managed to save when I took off the stock bars.
Hey - that looks really nice Solomoriah!
One thing to note - many (most??) aftermarket master cylinder mirror mounts are actually RIGHThand thread (i.e. the usual direction).
I think it may be only OEM Yamaha masters that have the odd-ball LH threaded mirror mount.
And you can't bring a "spare" everything. The best you can do is to be prepared and equipped to do "damage control". Good luck, and keep your face in the wind. 
