Many users have managed to replace the battery within the Pacemaker device.
The new battery must be able to handle 1A charge current. Other than that – any battery with 4.2V charge voltage and built in protection circuit should work.
There’s a great forum post here which explains the process of changing the battery within the Pacemaker device.
The orginal battery size:
Length: 48mm ±1 mm (not including tab length)
Width: 30mm ±0.5 mm
Thickness: 7.2mm ±0.3 mm