I guess the engine is stone cold now, but if you can turn it over and suck penetrating oil in to the cylinder through the intake then you could possibly brake up the rust on the inside.
Blaster 16-PB Penetrating Catalyst - 11 oz
At least spray this down in to the spark plug well and let it sit for a day so the oil can sink in to the threads. IF IT WERE ME . . (and I take no liability for this) I would heat the oil up in a cup etc to about 130 - 140 degress and pour down in there (before it cools down again) and let it soak, but that's me and I could be certifiably insane for all you know.
When trying to break the plug loose apply pressure very slowly and hold it, let it come loose on it's own time and don't put muscle in to it or you can kiss those threads goodbye. If it starts to turn reverse direction and go back and forth slowly with as little force as you can gt by with and take your time. An hour to slowly remove that plug is better than an hour to re-thread that hole.
And when you do finally get that plug out, try to swab down in the threads on the head with a clean cloth and spin the wad counter clockwise, clean out that cylinder well before you put the new plug back in.
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