EGT and Mag
If you have EGT measurement on all cylinders and they all drop to zero when you switch the mag this almost certainly indicates magneto trouble or the connections from the magneto. If only one EGT drops then it is most likely a plug.
If on the other hand you have a single EGT gauge then it will be connected to a single cylinder exhaust. So assuming you have 4 cylinders there is a 3/4 chance it is the magneto that is the problem, and a 1/4 chance it is a plug.