1993 Mazda 929 Oil leaking into spark plug wells

My 929 is now misfiring and often shaking when at idle. I found the reason when I replaced the spark plugs. Two were wet with oil, and two I couldn't replace because they were stuck.

How much do you think it would cost to open the engine and replace the gaskets/seals leading to the
spark plug wells? Would this be classfiied as an engine overhaul?

Thanks !