If you could post the wiring diagram, or post a link, that would help (for future needs also). Also, the engine model and serial / date code is stamped into the top visible side of the valve cover.
with the extensive tear down of the engine to do the head gasket, I would look at the wiring to see if maybe a power wire from the stator got pinched along with the kill wire on the magneto. A quick test would be to disconnect the kill wire off both magnetos and try shutting it off and see it blows a fuse.
To save yourself a few dollars in fuses get a 12v, 20amp circuit breaker to use for testing. It will "open" when over-current and then reset once the short is removed
(key on?). Of course, once you disconnect the kill wires, you won't be able to shut down the engine if running. Just reconnect them and then you can shutdown by the key.
good luck, post the other info.
12v 20amp circuit breaker (link)