Sparking Plugs with an inbuilt resistor are bad news for a magneto. As a result of the resistor the magneto has to generate a higher voltage to overcome the resistance of the resistor plus the resistance of the air gap. This increased voltage puts a greater strain on the armature windings and their insulation. This will eventually cause them to burn out and fail.
I would also advise against using resistor plug caps for the same reason.