[InFlight]

Buying sets of odd module metric gears will be difficult due to availability, and will be more expenive than standard 20DP / 14.5 angle gearing offered by Vex and Andymark.

I would suggest removing the stock pinion and replacing it with a standard gear would be the best long term approach. You can order spur gear stock and cut it to length. Center drill (in a lathe) to fit the shaft. I'd suggest the addition of a small tapped side hole for a set screw rather than trying to achieve a press fit onto the shaft.