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The broken gear has not been fully meshed with the starter pinion. The starter seems like it should be moved across left or right so it meshes full width of the gear teeth. I doubt that alone would break gears repeatedly though, or not that fast anyway. Too much ignition advance would strain the gear. I'd check the ignition timing and advance, and try to ascertain how much it retarded as it went down to low revs. The cdi is really old and it probably gets advanced timing, and more ignition advance as it revs than it's meant to, if the air-gap on the pulse coil is too small.,