I am not familiar with QMK but I have converted mine to use a Teensy++ and wrote a simple firmware for that. The nice thing about the Teensy++ is that it fits into the socket of the original 8051 (clone), you only have to crosswire or remove a couple of pins.

This post inspired me to modify my own in a very similar fashion: https://geekhack.org/index.php?topic=12212.0

Teensy++: https://www.pjrc.com/store/teensypp.html

And this is the firmware I wrote and still use: https://github.com/imarko/dhteensy

I think there might be some better versions out there but I wrote this to work just the way I wanted so never bothered with anything else.

I don't have one to sell unfortunately, my spares may need to last me a lifetime :)