DYS BE1806 are pretty common in beetles. I get mine off ebay. I had issues with the 25D gearboxes (pinions coming loose or gears not meshing/exploding) so I pair them up with 22mm 19:1 planetary gearboxes (https://www.ebay.co.uk/c/2022384158).

I buy two donor planetaries per DYS motor, transplant the entire motor shaft from one donor motor into the DYS motor and replace the plastic first stage with the metal gears from the second stage of the second motor.