My percussion massager (DDVWU), though only a few months old, stopped working. It no longer took a charge and refused to power on. I first disassembled the massage gun. All of the PCBs appeared to be functional. Suspecting the battery was to blame, I searched online to find a replacement rechargeable lithium battery pack. Unfortunately, a replacement battery was nearly the price of an entirely brand new massager. As a result, I replaced the battery with a wired power supply. The massager now requires one to plug in a wall wart to an AC outlet, and the gun remains tethered via an electrical wire to the outlet. This is fine for me, though. I won't have to worry about another battery permanently failing. See below parts needed for your project.

Voltage adjustable power supply (3 amp):
https://amzn.to/3vhSsl6

Consider instead a larger amperage power supply to ensure your unit does not stall with high force applied. Some massagers reach 6+ amps.

12 volt, 7 amp power supply:
https://amzn.to/3IT1U19

Generic power supplies:
https://amzn.to/4cq83zw

Percussion massager:
https://amzn.to/4cvog6F