You could use this battery from Treats, I keep it under my Streetmate's seat where the lead acid battery used to go, it's a bit less than 1/2 the size and 1/4 the weight of the original:
https://www.treatland.tv/trail-tech-sealed-12V-battery-pack-p/trail-tech-battery-040-batt3.7.htm
I use this to charge it at home:
https://www.all-battery.com/nimpackcharger_01025.aspx
And there's loads of different shapes and sizes of 12v packs here for less money/Ah than the trailtech:
https://www.all-battery.com/12vnimhnicdbatterypackseries.aspx
Only thing to watch out with NiMH packs is you have to recharge them if they've been sitting unused for a long time, but I guess that's true of li-ion as well. Unlike li-ion you don't have to worry so much about running them flat. (edited)