Mine lost spark until I charged the battery. I figured out that the voltage regulator/rectifier was bad by using a multimeter. I tested voltage from the stator at the connector the regulator plugs into, white wire to blue wire. You should see at least 12 volts AC. It should go up higher than that when you rev the engine (mine goes up to 30 VAC). If that's there, you have a good stator. Then test the regulator/rectifier, which would probably be best tested by connecting your multimeter to the battery with the engine not running, and note the voltage. Start the engine and rev it up. The voltage should go up. If it doesn't, you need a new regulator. The inline fuse is part of this circuit, too, but nothing works if it's blown.
For you electronics nerds like me, I test the regulator output at the battery because the raw feed from it while disconnected from the battery can confuse some multimeters because it's pulsed DC, not constant DC.
