[asid61]

You can make a 1 stage shifter f you use 4 inch wheels and can machine a couple gears down to length.
Basically, have an 11t pinion and a 14t pinion on each CIM shaft and have them gear to two gears- a high speed and low speed gear. Then shift between the two gears.
That's a single stage version, but you can only get a spread of around 1.4x due to the maximum size of the gear being limited to the size of the wheel. Adding a second stage would allow you to have massive spreads.