I'm sure someone who had dealt with this will come on and give you some advice.
If it were me, and in the absence of any better ideas, I'd get a new plug, then use a dremel to cut a deep slot in the head. Then I would put the bike against a wall so I could push hard against it. I would use a small prybar or similar that fit the slot nice and tight, push hard and turn.
Hopefully someone will have a much better and proven idea for you!