Actually, someone did do a good deep dive into the hardware of the things and was easily able to get root access which would in theory allow you to do whatever on it, including add new games to the system. You can read more info here if you like:
https://www.reddit.com/r/Arcade1Up/comments/a02870/hack_original_board_not_just_replace_w_pietc/ <
But as noted above, it's possible to put a Pi (or really any PC with USB ports and HDMI, if you wanted to) into the Arcade1Up cabinets which is 100% the better solution, because the SoCs used in the cabinets are garbage (a 10 year old
Allwinner A13 in gen 1 and 6 year old
Allwinner H3 in Gen2) and wouldn't give you great performance on most games you'd want to add. I have a little arcade stick AIO that has an Allwinner H3, and the performance on even PS1 games is basically terrible.
To add a Pi (or PC), you'll need to spend at least $80 on components (not including the PC/Pi and any accessories like SD card or cases, which will set you back another $60+ depending on what you get), including:
1) Replacement buttons+sticks with a new USB encoder < Usually around $30-$60, depending on how good you want the buttons
2) Audio amp for the speaker < Like $5-15 depending on what you get
3) An HDMI controller board to connect the Pi to the screen < $25-$30 if you want a decent one.