Simple? stop and plug your charger in periodically.
Next best? Get a regular generator that's small enough to carry with you. Ruggedize the charger it comes with (many will break from vibrations if carried around). Plug it into the generator's AC outlet, and the vehicles' charging inlet. Start the generator. Ride.
Now you can ride constantly until you run out of gas.
You can build your own generator for it, but realistically it's tough to build a good regulator circuit to control the speed and output, especially if you are varying the speed of the engine itself to drive the scooter.
The simplest way to do that is to use an engine off of something that already has a generator/alternator built into it, which is usually meant to charge a "12v" lead-acid battery. Then you could use one of those cigarette-lighter-socket AC inverters, powered off that generator, to run your regular charger, hooked up to your charger inlet, just as above. The catch is that you may alos have to add a small lead-acid battery at the generator/alternator output to smooth the power coming out.
But you might wanna check into your local laws, and see if they enforce them, cuz some places (like Phoenix) motorized scooters like that are not legal. Some people get away with electric ones cuz they're quiet and don't attract much attention, but the gas ones are pretty loud (some can be heard for blocks) and often attract unwanted police attention. People ride them anyway, but they do get confiscated sometimes.
