How do I test a lawn mower magneto?
Do you see the single spade terminal that is on the coil? If that is grounded, the coil won't fire. Pull the engine shroud off and disconnect the wire from that terminal. Next check the spacing from the coil to the flywheel is good. I like to use a business card but a thin piece of plastic works too. When the magnet is facing the coil the gap between the coil and magnet should be even and about that much.
Get a good new spark plug. Put it on the coil wire. Touch the threads of the spark plug to the block and have a second person crank it for you. This way you can easily keep an eye on the spark plug. If it does not spark at this point, you have a bad coil.
If it sparks then plug the wire back in. If the sparking stops then there is a short to ground on that wire.