I'm amazed that after all these years no one has wired this up. The MPC One (OG and plus) will happily run on a 12v 3-cell LiPo pack. I've had good luck using these KBT 12v 2600mAh packs either attached with velcro on the outside or inside in the bracket underneath the pads.
https://www.amazon.com/dp/B0FD9J9KZY Adding the missing Schottky diode onto the main PCB lets you run one of these packs in parallel with the original external adapter. I added a dedicated barrel jack to the side for charging. I treat the internal battery more like a UPS than my main power source, and still use an external pack most of the time.
One of these days I'll get a logic analyzer on an official Akai/Dennon battery pack and see if I can get the built-in battery gauge working
