Spark Plugs 6 V verses 12V

not sure about magnetos---never worked with one

However the maximum voltage actually generated by the coil and "seen" by the spark plugs is a function only of the electrode gap distance and of the nature (pressure, temperature and composition) of the gases in the gap. This is because once the gases in the gap ionize and the spark begins, the spark path is essentially short circuit across the electrode gap - very little resistance, hence a very small voltage drop.

Bottom line: Doesn't matter if your electrical system is 6, 12, 24 or 28, or even 0 volts (magneto) - the spark plugs won't know the difference. You use the same plug, and expect to see the same performance and plug life regardless of system voltage!

Regarding advantages of 12 V has over 6 V is 12 V, the only advantage 12 V has is it lets you use a 6 volt coil with a ballast resistor. Then by arranging the starter switch so it bypasses the resistor while the engine is cranking, you develop an extra hot spark, just when it's needed most (since you are now feeding the 6 V coil 9 or 10 volts during the cranking cycle). This offers no advantage to a low compression tractor motor. However the ballast resistor setup was necessary to insure reliable starting of the high compression automobiles of the late 50's/early 60's. These motors required a lot of battery power to crank, and also needed an extra hot spark to fire the highly compressed fuel/air mixture.