If I was in that situation, I would drill and tap the welded-in plug with a size that is a little smaller.

The original plug is M14 I believe which is a little bigger than 1/2 inch.   Drill and tap the rigged plug(In situ)for a 3/8 plug that you can buy at Pep boys or similar.  Bear in mind that the tapping drill size for a 3/8 plug will be a bit bigger than 1/4 inch so dont drill it 3/8.

This will be a bit messy and awkward, i.e. drilling from underneath till the oil comes out.     The oil flow will be slower than before but it should at least be a permanent fix.

Steve Cotsford