I replaced my plugs and coils at 196,000 miles

I replaced my plugs and coils at 196,000 miles. Apporximately 400 miles later I blew out a plug. My "mechanic" tapped a new hole and replaced the plug and coil. Six months later, it blew out again. They made another "attempt" Fifty miles later and the plug blew out again. I love my convertible and do not want to ditch it. I want to eventually supercharge it (I know the internals need to be forged). In lieu of spending $38k for a new Mustang, I thought I could either try and tap the plug again myself, I also thought I could permanently epoxy the plug into the cylinder until I could get work on rebuilding the engine, Since it has been tapped three times I do not know if I can do it again. If I replace the cylinder would I not have to replace all the gaskets since it such an old engine? Lastly, I cannot find a mechanic in my area who wants to tackle a rebuild. If I cannot go that route I thought I could put a reman stronger engine in it with forged internals AND supercharge it. What engine, Mach 1, the new Coyote 5.0, etc, would fit in the car with a minmum of trouble that I could avoid a rebuild on my old engine and supercharge it? Or is it not worth the trouble at all?