Well, I got it apart. I heated up the top cap a bit with a propane torch, then with a screwdriver, popped it off. It was just held on by annular deformation, sort of like a soda pop bottle cap. Turned out the copper rivet was not important, just holds a spring finger tab, no need to remove it.
Next, a magnet removed the steel slug.
Next, unsolder the two ends of the coil wire and it comes out.
Next, there is an internal expansion spacer ring to come out.
I got a small screwdriver in there and scootched it out a little at a time, kind of like removing a piston ring in reverse.
Next is a steel plate, which was tight in the housing, so I tapped lightly on the copper studs until it came out. That was tricky because you are working against spring force.
There is an insulating cup and a couple of insulating washers in there so you have to be careful not to wreck them.
Here are the offending parts. The copper ring is insulated from the center shaft. The center shaft and copper ring are pushed down by the steel slug when the coil is energized, and the ring makes contact with the two copper studs.
Every time you run the starter there is a spark across these terminals which makes a little dirty spot. Eventually you have no more clean spots and no current will pass to the starter motor.
It looks like I can clean these up and reassemble the thing. The trick will be to ensure that nothing touches ground that shouldn’t. The insulation around the little side terminal is a bit damaged so I may have to come up with something better there.