Summary

Four standard 37 mm diameter DC motors are required.

These are two types of special metal parts. Processing is required.
https://www.monotaro.com/g/00428592/
https://www.monotaro.com/p/4211/1273/

This is a special wheel.
https://www.thingiverse.com/thing:4808618
https://www.thingiverse.com/thing:4808637
https://www.thingiverse.com/thing:4808827
https://www.thingiverse.com/thing:4808821
https://www.thingiverse.com/thing:4808952