IF your battery is GOOD it should hold a charge and your battery is charged by way of the motor to the voltage regulator.I've seen battery's go bad in a week and the one you put in could also be bad.get a known good battery if you can.
If the voltage regulator goes bad and doesn't pass along about 14 volts back to the battery it won't hold a charge because It's not getting enough volts to the battery.
So assuming you have a GOOD BATTERY I would look into the voltage regulator/rectifier.
The source in the motor is the stator....Stator produces voltage to the regulator that feeds voltage to the battery .
Feel free to correct this but as far as I know this is how things work on moto all over the world.
Good luck
You can test the regulator with a digital meter at the battery with the motor running digital meter leads on the battery you should see about 13.5 volts and rev and hold the rpm's at 4000 and you should see slightly higher about 14
You're just left with yourself all the time, whatever you do anyway. You've got to get down to your own God in your own temple. It's all down to you, mate.
John Lennon