If it were me I would stick a square piece of metal in there and tig it to the plug. Then remove it. Replace it with a new one. I. Have never had a BORG but I’m assuming that the plug is cast iron so therefore weldable. I would use Tig to keep the heat down and not screw up the aluminum housing. Yes you could fill it from the drain with a pump. Look up fluid amount the add an ounce of two for what you would loose getting the plug back in. Hope some one smarter has a better idea. Best of. Luck.