[Rob Stehlik]

I highly recommend you measure the shaft spacing in the Fisher Price gearbox and use that in your custom design. I recently counted up the teeth and calculated the pitch diameter of the first two gears (they are 32DP by the way) and made a gearbox based on those numbers. The gears were way too tight! After double checking my numbers and not finding any mistakes, I measured the plastic gearbox, and found the spacing was much larger than expected. I suspect the motor pinion has been profile shifted, which directly changes the center distance.
I'm happy to share my design if you want. Send me a PM.