the mini X uses a motor shaft that extends the length of the car. Kinda like how the xmod uses that shaft that goes the length of the bottem of the car and connects the front to the back but through the motor instead of the gear drive. Ill search the mini x forum and find the dealer website so you can order one. You could just cut it to whatever length you want.
As for pinion gears being different sizes yes their are different size pinions but i believe they differ in diameter though. If it had fewer teeth but was the same diameter then it wouldnt mesh properly.
The motor shafts and pretty small so cutting a key knotch would be pretty hard. A dremel might do it but you might try making the ring out of something like brass and soldering it on.
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