Start the engine, step up the throttle, use a multimeter to measure the voltage at both ends of the battery should be between 13.5V-15V, if the voltage is too low, replace the rectifier and check whether the magneto wiring harness, rectifier, etc., is connected, use a multimeter to measure the AC voltage of the magneto stator, step up the throttle when the AC power supply headlamp type should reach more than 20 volts, The magneto output of DC powered headlamp should reach more than 50 volts, otherwise the magneto coil may be faulty.
Unplug the battery insurance to see if there is a light, there is charging, there is no immediate fire is not charging.