I would not use the J8C plugs, unless you plan on idling all day long, they are a bit too hot for high speed running. I’m sure the boatyard replaced the coils if they were cracked, kind of hard to miss that. But, they may have misrouted ignition leads under the flywheel, which may have eventually rubbed through….
The fuel pump is on the top bypass, so an issue there would be affecting the top cylinder….You mention that the intake looks untouched, so I guess issues there are not likely. What about the fuel pump pressure port that comes off the intake, is it plugged off securely? You could try decarbonizing, but that is probably not an issue unless you have been idling/trolling excessively.
Again, any evidence of water on the plug(s)?
I’m not sure about whatever lucas oil blend you are using, you mentioned using some generic stuff also, perhaps that is an issue.
Finally, this is a relatively low compression engine, there is probably no benefit from using the higher octane fuel…But, I realize you are using it mostly due to the lack of ethanol.
Finally, how does the engine run before the lower plug gets fouled? Idle OK?
