This gearbox motor came out of a rover that took a too steep of a fall and landed on the front wheels and this is one of the two front motors that have since broken. Disassembly showed that two of the gears in the gearbox now have broken teeth, and that's why it didn't run smoothly anymore.
And these teeth are far too small for most home makers to replace and fabricate. It's much too small for most 3D printers, even it had the strength. So I went online looking for replacement gears.
I can't find just the gears. Then I went and looked for the gearbox and I can't find just a gearbox. The motor and the gearbox motor encoder are good, so I didn't really want to buy the entire unit.
So the next best thing is Pololu also sells the same size motor with the same gear ratio gearbox without an gearbox motor encoder. And that trims some costs off a replacement.Some rover builders decided to upgrade to bigger and beefier gear gearbox motor encoder and that's a great option too if you have the budget for it.
But if you only want to spend a few tens of dollars, we might be able to just swap out the gearbox motor encoder. It just depends on Pololu using the same gearbox motor encoder of the same vendor so they're interchangeable, but these are mass-produced items that odds are good.