Magnetos put out marginal voltage at kickover speeds, so it can help to use a fine-wire plug - these will fire at a lower voltage. But as noted, you want a non-resistor plug. Unfortunately most fine-wire non-resistor plugs have been discontinued, since cars no longer use them. They can often still be found, though. NGK fine-wire suffixes are 'GV' for gold-palladium, and 'VX' for platinum; I've run B6EVX on a Vincent twin with good results.