It does sound like the scooter's alternator isn't making sufficient voltage to keep the battery charged.
So, the 1st question is do you have DC voltmeter to measure the voltage on the battery ?
If yes, then charge the battery fully.
Then attach the voltmeter and record the voltage as you change the RPMs.
Remember to be sure that the rear wheel does not touch ground when the engine is at higher RPMs.
The voltage on the battery before you start the engine should be greater than 12.0 VDC
With the engine operating the voltage should go up to about 14.5 volts when the RPMs are about 2500 (a guess).
No need to rev the engine about 3000.
If the voltage does not climb towards 14 VDC, then the chaeging system is not working.
There are 3 parts to the electrical system:
(1) alternator, which makes AC voltage
(2) Rectifier, which converts the AC to DC voltage
(3) Regulator, which keeps the voltage to below 15 volts.
Figuring out whether #1 or #2 are faulty is more difficult.
One would need a service manual for the location of the parts and the values that the tests should show.
Is there a dealer nearby ?