I think the plug is cast iron, at least that's my first guess from looking at mine.
Someone brazed a rod on mine when the corners rounded off.
If I was in your situation I'd weld the corners enough to remove it, finish welding the corners and file or mill it square again; as good as new.
I doubt you have much luck finding a new one, but it's worth asking around.
If it comes down to it a new one could be made.
Grigg
