Heat the plug until it's almost red, and apply some candle wax or some other type of paraffin such as the jelly-sealing stuff that's available in blocks at most grocery stores. Hot wax seems to penetrate seized threads much better than most types of oil. I do that when trying to remove oil gallery plugs for a pass with gun cleaning brushes during engine rebuilds.

It takes a lot more Propane than acetyline to make enough heat for loosening threads. My "weed burner" wand and a 5 gallon Propane cylinder comes in handy for that little chore, if the area to be heated doesn't need to be precise.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!