My advice would be to get the drain plug out as best you can - air tools / fire / mole grips / whatever. Use force and aggression on the bolt - but preserve the oil pan.

It sounds like you are going to be "stuck" with a knackered drain plug what ever happens.

Once you've removed the plug (OK easier said than done perhaps?) remove the oil pan.

Inside the oil pan you'll see that the threaded hole in the pan is just a flat bit of steel with a thread in it. This is tack welded onto the oil pan.

Grind off this flat bit of steel and weld (or get someone to tack weld) a nut or another strip of steel with the correct threaded hole for a new drain pan bolt.

Buy a new gasket for the oil pan.

Buy a new drain pan bolt.

EDIT:-

Alternatively buy a new lower oil pan!

__________________
1992 W201 190E 1.8 171,000 km - Daily driver
1981 W123 300D ~ 100,000 miles / 160,000 km - project car stripped to the bone
1965 Land Rover Series 2a Station Wagon CIS recovery therapy!
1961 Volvo PV544 Bare metal rat rod-ish thing

I'm here to chat about cars and to help others - I'm not here "to always be right" like an internet warrior



Don't leave that there - I'll take it to bits!