I did both the magneto nut and the boss recently on what looks like the same motor. If you havent got a rattle gun then the trick with both the magneto nut and the drive plate nut is to put a golf ball etc in the cylinder then put the head back on. Now the crank cannot turn. and a spark plug socket 13/16 is good for the magneto nut. To remove the magneto itself, put a lever both sides and push down. If the boss is seized onto the output shaft, use two equal hammers. Hit it on opposite sides at the same time and same force.
