If de battery connection to the motor is only power, without any control bus (CAN or anything else), the the easiest replacement for a battery not available as spare part is to use batteries for professional portable electric tools from a known, reliable brand like Makita, Bosch, Milwaukee... there are several voltages available, 12v, 18v, 36v.

These batteries made to be used outdoor are waterproof, rugged and very reliable. Since they are made for professional use, there are fast charging systems for them.

To have a higher capacity, a simple solution that can fly is to use two 12v batteries for a 24v e-assist and 2x18v < 100Wh batteries for a 36v e-assist. Its easy to find on Aliexpress a tool side connector for these batteries.