The timing doesn't change. The only way to change or adjust the timing on our bikes is to rotate the coil, relative to the position of the crank. If you've ever been around an old boat motor, you'll notice that part of what happens when you crank the throttle is that the entire magneto plate rotates to advance or retard the spark.
If I had access to a spare simplex motor, and lots of time, I'd like to see if a boatmotor magneto plate & flywheel could be adapted.
Mike Sal