There is a hall effect sensor fitted to the side of the the crankcase to tell the spark when to spark.

a lot like this


I don't want to fit a nose ring as I'm worried that I will either break the hall effect sensor if I have the ring too close to it when the engine power pulses or, the gap left in the nose ring to allow free rotating of the engine and hall effect sensor will negate even having the nose ring there, i.e. instead of an O shape it will be a D shape and therfore useless at keeping the engine in line.