Looks like you have a bushed motor. Just wanting to upgrade your battery?
Ignore the watts and focus on the ah. You have 12ah now the same 12ah Li battery will get you more distance. Get a 17ah 48v li and you will get much more distance.
More reading.
Battery Primer:
Volts (v) is speed and Amp Hours (ah) is capacity or distance. (not the same as discharge amps (a))
12v Lead battery is 6 cells in series. 48v SLA would be 24 cells in series. Typical 48v Li is 13 cells in series.
4x12v 12Ah SLA batteries in series
Each battery is 12v 12ah. When you combine them in Series (s) you add the Voltage together and the (ah) stays the same.
If you combine them in Parallel (p) add the ah together and the volts stay the same.
You will get more out of a Lithium (Li)battery, can use about 80%. SLA should less than 50% of capacity. Lithium batteries are much lighter.
Note: Not all Ii batterys are the same they have different chemistries that would alter the voltage range.
48v Li-battery had a range of 40v-54v. Need to make sure your controller can deal with that. 48v SLA 46v-51v range.
If your controller can work with a 48v Li then figure out how much space you have and find a battery that will fit or have one made.
Related links:
Want to change from SLA to lithium