The files for a Frame and PCB that adapt a LiPo battery for the OP-Z
With the Frame and the PCB you can plug in any 3.7V LiPo with a PH2.0 connector that will fit. The largest battery I found that can fit is a 503048, this turned out to be a 800mAh, the original was 740 mAh. To assemble this glue the PCB into the frame, insert the battery and then wrap with Kapton tape to keep everything in place.
Here is the link to the official replacement battery that is no longer available. The battery used to retail for $59.99 USD
https://www.ifixit.com/Guide/Teenage+Engineering+OP-Z+Battery+Replacement/128285
https://www.ifixit.com/products/op-z-replacement-battery
If you want to buy me a coffee check out the link below, if not, no problem
https://buymeacoffee.com/buxelectronics
If you don't want to make this yourself I am selling them on Reverb