Try opening the plug gap about 20% above recommended. Also try removing the resistor in the plug lead cap if that doesn't help.
Going to a little hard as if the ipls are right here. They don't show external resistor plug cap being used with the coil and the one in the resistor spark plug is none removable.
The stock plug for the GCV135 was a NGK BPR6ES. To decrease resistance in the ignition system you could replace it with a BP6ES but beware that the RFI will increase considerably from the increase spark strength and may affect nearby electronics. It would also increase the spark advance (timing) slightly due to the decrease time it takes the coil to produce enough voltage to jump the plug gap.
Opening the gap would only delay the spark timing slightly as the coil must develop a higher voltage before it can jump the gap which actually could damage the ignition coil although these coils should able to handle a .035" gap. An alternate way of changing spark timing would by adjusting coil to flywheel air gap clearance but even that has it limits. This affects the coil triggering electronics by changing the induced voltage timing.
