If that was my bike, the very FIRST thing that I would do is make VERY sure that the battery is fully charged and that it doesn't suffer too much of a voltage drop under load.
Try measuring the voltage across the fully charged battery with the headlight on when the motor is not running. If it drops below 12 volts, then try another battery.
If you do not have another suitable battery, then perhaps connect up a good car battery in parallel using jumper leads.
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[color=#808080][size=9]Yes,.. I too know how to waste Time and Money,...