ahh ok, have it in 1st and get it to TDC where it gets compression and you've gota give it a real quick twist and it will come undone, if ur struggling you can try:
jamming a flat blade screw driver in the magneto to hold it
or
have someone hold the rear brake on while the bike is on the ground
jamming a flat blade screw driver in the magneto to hold it
or
have someone hold the rear brake on while the bike is on the ground