These SLA cells will fit into the existing bag with plastic inserts, and can be connected with the existing wires.
Note: if you can find a local source or discount online, the specifications for replacement cells are 12V 12Ah deep cycle SLA batteries.
These standard batteries can be found with a search like “SLA 12V 12AH battery deep cycle”. This will point to many stores for batteries to power your ride.
Some riders have used larger (20Ah) and increased the range at the trade off for more weight. It’ll require some mechanical effort to secure these larger batteries in place.