The Sage battery on mine was just "naked" wires and I can see it has a taped in integrated controller, so likely any cell with the controller built in will work. Very basic soldering iron skills needed, though cutting existing wires (one at a time!) is possible, and some sleeving.
The front has everything except the PCB for the 4 contacts on it, so pop open at other corner and be careful of the flexible pcb ribbon to the small PCB mounted on the rear casing. An old credit card or plastic pry tool just to start. Easily pops back together.
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