Very cool scooter. I haven’t seen any of these in the wild, but seems very mod-able.
Max voltage you can go to is going to depend on what controller you have inside there. Seems like most people are reporting that it can handle up to 60V, like you mentioned. Any lithium-ion battery you install is going to be way better than the sealed lead acid ones you have in there already. Lithium manganese (INR or NMC) is a pretty popular chemistry for scooters that delivers good margin of safety, high current output and reasonable longevity. You can read more about them here.
As for which sprockets are going to give you fastest speed, not exactly sure. Once you get the new battery swapped in there, I expect you’ll have quite a bit more power overall. If you opt for the 60V battery, I’d expect the most torque, so you’d probably want a lower overall gear ratio so you don’t limit your top speed. If you go with 48V or 52V, then you’ll get less torque and might want a higher gear ratio so that you have decent acceleration. If I were doing this mod, I’d get the new battery in there first, and see how the accel/top speed are and adjust the gear ratio accordingly.
Give us an update once you get your battery swapped! I’m sure a lot of others would be interested in knowing how to do this mod.