there is normally an earth lead going down to the engine itself. Frame to earth. The pulse is created when the flywheel passes over the pickup, part of the stator assembly, which sends a signal to the cdi to tell it to send a spark. Could be the pickup itself or any of the wiring down there as well. Or the coil/coil connection/lead/plug.
I have changed the engine last week because the garage told me it was seized .. I had no spark from that engine and same with this one could both stator be broken ???