Summary:
After building a Robak 2 for my nephew using robaki's Reduction Gears, I was reminded of Duke Dok's chassis and my not-so-great decision to use a 3900KV motor.

This gear set is designed to reduce speed to approximately one-third. The additional bearings required are the same size as those already used on the rear of the original chassis. To accommodate printing inaccuracies and prevent binding, 0.4mm were added to the optimal center distance of the gears.

Thanks to Robaki for providing the STEP files for his design in the Robak Discord.

Onshape assembly:
https://cad.onshape.com/documents/efd634b925a8c3fb3f932cc1/w/752ca0fa2879f907ecae846f/e/91b3f1af997314ec04e679e9?renderMode=0&uiState=67b39f1cdc1ffe21f0b7d8de

Hardware:

"Happy printing, and God bless, my friend."
— Bob Ross