RD suggested a TORX bit and these are more successful than the tapered extractors that in effect expand the offending thread against the housing. I would add that heat with these aluminium castings is another trick to break the bond of alloy and steel, along with any other sealer... Shellac from way back or Loctite from not so long ago.
The plug is part of the anti wet sump valve and a "Do Not Disturb" warning meant it was not to be messed with by your average Joe, so it is not listed in my parts books at first glance. Spring and ball are available, search by description. Plug can be made from a slug of BSW threaded bar, or a cut down BSW bolt as suggested by bergs.
Clearing that crank oilway is the single most important aspect of rebuilding that engine...... methinks.
If you go for a modern oil, a filter in the oil return line is essential to remove particulates before they go back into the feed side and once more settle out in the crank.
Swarfy.
