One way to lock up the engine so you can remove the crank screw or in your case the magnet nut is remove the clutch side cover and put a shop rug into the gears teeth so as you turn the nut to remove it the rag will jam in the gears teeth so you can remove the nut this will work on all the nuts on the engines shafts. you can also take off the sprocket cover and hold the sprockets shaft with and end wrench. the rag trick works the best in my opinion. One of the guys said to pull out the sparkplug and put cotton clothsline in the hole this will also work but try to get the piston to almost the top of its stroke so some of the rope will not fall into one of the ports. I don't know if you could get that unlucky but I'd not want to find out that the rope became shorter after being in the cylinder.