Do a search, removing stubborn screw fixingsis a bit of a regular. I would not use an extractor, if it breaks you're in real trouble. Try a torx bit that's just a bit too big and hammer in in. This will cut some new shoulders on the recess, and you can have a go at unscrewing the plug.

Any screw in device will leave you with a hole and hence a leak.

Is there enough of the plug exposed to get some mole grips (vise grips?) clamped on? This should give you more leverage.

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Cheers, Neil