One thing I have seen done is two extra spark plugs fitted by angling them in on the insides of the combustion chambers. If you did that , firing each pair using double-ended coils off a Boyer, you would know when a plug died in service. The thing I mentioned about coke in the combustion chamber on the side away from the plug, I suggest is significant. There is a limit to how lean you can run the motor and it is set by reading the plug, but you only see the side which is near the plug. Around the other side of the piston crown, there must be a lot of mixture which does not convert to energy.