Consider the stock battery's specifications and know the replacement battery will have to have similar specifications.
If you calculate what the replacement battery must operate, maybe you can calculate a smaller battery which can still suit your purpose. In other words, if you don't use directionals or a horn, that is less current. If you are running LED lights, they use less current. If you don't travel far, that is less current, but may require a trickle charger to make sure the battery is fully charged before each use.
Keep in mind, the physical size of the battery has nothing to do with the capacity of the battery.
Lithium batteries are small, light and very, very expensive. Not all batteries can be charged with the same trickle charger. Your replacement battery may need a special charger.
One suggestion is to buy a brand name/quality battery. You DO get what you pay for.
Good Luck
-Ride On