These handhelds can be actually programmed with another game, by further testings, provided if you get the registers (especially the $41xx OneBus-related) correct.

However, you need to dump its original ROM and try to work around its bit-swaps and the TFT initialization routines.

As writing the ROM dump analysis and the such is a bit too difficult inside the log entry, I have separately written a Github page on this one: https://nyh-workshop.github.io/Custom-ROM-Sup-Game-Box-400in1/