This is my example of converting a Takahashi NJP mount to GOTO using OnStep.
I used the original mounting hardware and gears from the existing PD5XY motor drive system. Reduction ratio is 1:2.66667 for RA and 1:5 for DEC.
MCU is Arduino Mega 2560 with DRV8825 motor driver and ESP-01. I soldered my own shield and recently added a DS3231 RTC with the help of Howard.
The stepper motors are: 200 step NEMA17 (60mm length) for RA and 200 step NEMA 17 (42mm length) for DEC. I used a DC-DC step up module to run the motors on 24V.
3D printed motor housings, mounting plates and controller case.
Attached are some photos.