Just superglueing 10mm micro motors (6mm diameter) with wheels superglued directly onto motor shaft (both canibalized from small solar toy cars) with sone motor controller, lipo and MCU is all that is needed to get fast driving mini robots:
This is latest modules based on Arducam Pico4ML-BLE which has 320x320 camera, 160x80 display, 9-axis IMU and bluetooth module, for 2-wheel balancing (here with little demo displaying measured roll value on analod display):
These are the other robots, from top to bottom, left to right:
Tiny2040bot, Picobot, Toycarbot, Pico4MLcbot, c18bot (fits completely into cube with 18mm sides; later c16 based on 10x11x2mm ESP8285 model ESP-01F), Tinier2040bot (with MCO as well as motor driver reduce in size with Dremel):