If it is TRULY a Magnum 18 HP, it doesn't have a "coil", it has an "ignition module" that is basically an electronic version of a magneto in that it is self-contained and requires NO battery/12 Volt power for operation.
If it is REALLY losing spark, there's two things that will cause this... a bad module, or something that is loading or shorting the "kill" wire.
Have you tried disconnecting the wire from the "kill" terminal on the engine to see if spark returns?
(WHATEVER you do, DO NOT apply battery power to the "kill" terminal! You will let some very expensive smoke out!)