Doug,
The only source for a new gear that I know of is Vahan Dinihinian, he advertises in the AMCA magazine. He can supply the fiber gear with rivets. These will fit S-2 and NS-2 mags. It's a careful job to get it installed and centered correctly. They aren't cheap, but its well worth it, I know first hand......
On the 2018 Cannonball, we were riding the 1924 Chief with Princess sidecar. We made every mile, every day, except one. Seems the S-2 Magneto gave out just before we entered Glacier Park. I had every part with me to fix the mag, except one and that was this fiber gear! When I opened up the mag, it reeked of burned bakelite, an aroma never forgotten! Every tooth on the fiber gear was chewed off by the steel drive gear. We got hauled in and had to ride the bus of shame into the parking lot. Easy repair once I got the spare out of the truck!
So, what happened? Seems the oil reservoir for the rotating steel sleeve that the fiber gear is attached to got plugged up. The bronze bushing and steel sleeve ran dry, causing the sleeve to start dragging in the bushing. Once that happened, it chewed off a tooth or two, moved a bit more, chewed those teeth off and so on, until all the teeth were gone. For the ride, I simply removed the 3 screws that held the "spider", as I call it, in place and replaced it with a spare assembly that I had in a second magneto. This is the steel housing with bronze bushing and oil reservoir, plus the rotating sleeve and gear that turns the rotor.
Once I got home and settled, I did an autopsy. What confused me was that I was oiling that bushing every other day, so why did it run dry? The S and NS mags are similar but the big difference is the NS mags have larger bushing, reservoir and a spring loaded oil cup for the reservoir, whereas the S mags have an open oil hole, small sleeve and bushing. Both have a bronze bushing with oil holes and grooves. The reservoir has some packing, or cotton waste inside to retain oil. There are dense cotton wicks that go through the bushing and oil the rotating steel sleeve. Seems all the old oil dried out, plus the carbon dust floating around inside from the brushes probably got in through the open oil hole and solidified, eventually blocking all oil flow to the bearing surface. Once the bronze gets dry, it starts to gall and snag, and the rest is history. It took a lot of effort, but I was eventually able to get the crud out of the reservoir. I tried soaking in parts cleaner, compressed air, carb cleaner, a flexible wire, pipe cleaners, everything. The crud just kept coming out. I finally used a water pick with hot soapy water and blasted the thing out. I was AMAZED at how much crud came out of that small reservoir! Once cleaned out, I replaced the wicks, added oil and all was good.
This was concerning to me. I've rebuilt a number of these mags over the years and have never seen a failure like this. I have a number of these mags restored, sitting on the shelf for future projects. I took the "spider" and fiber gear assembly out of each mag and inspected them. Every one was loaded with black, nasty crud. In the past, I simply put these in the parts washer and cleaned them up, blew them out with an air hose, added oil and all seemed well. Not so, they need a thorough cleaning to be reliable! I also replaced all the fiber gears at that time with new gears. I also built up a couple spares for my riders and I carry these on the bike!
When reinstalling the "spider" assembly, be sure the 2 gears are timed correctly. You will need to transfer the timing mark from the original fiber gear t0 the new gear and index this mark with the corresponding mark on the steel gear. Note the position of these before removing the assembly. The points need to be removed in order to see these timing marks meshing properly.
Hope this helps! Clean those things out and avoid a catastrophic failure of the magneto that will surely leave you stranded!
By the way, the steel drive gear drives the fiber gear. The fiber gear is required so the stem of the rotor can reach in and pick up the spark. If the gear was steel, it could short out if there was a crack in the rotor stem. Also, the fiber gear helps keep the noise down.