Welcome Novice razor modifier.
I think for that modest budget you will likely want to get a higher wattage controller and an additional 12v lead acid battery to wire in series so that you are up to 48v.
On Electricscooterparts.com you could find a 48V controller and an additional lead acid battery possibly even with the correct connectors. There is also a Facebook group called modified razors where people mess with specifically razor bikes so there is a little more knowledge on specific measurements or certain components that people have already put in that fit and work.
If it were me, I would start there, over volt the motor and see how long that lasts. If you are treating this like a toy and not a commuter vehicle, you should be able to get a little more fun out of that set up. Eventually you will probably burn out the motor but you should be properly addicted to this hobby by then and want to spend a bunch more money.
As a complete novice, take some time to learn how series and parallel affects power, take a little time to learn the wiring diagram of your bike and where there might be some improvements. For me, the mechanical stuff is easier to visualize and learn faster than the electric stuff so I treat it with greater caution. everything under 48V is low voltage though so it is less dangerous to play with.
