Also, do NOT drive the car w/o that air bleed screw installed. Once the system starts working properly it'll spew coolant out of there. If you can't find it, call your local dealer. You may need to buy the entire hose. Or, if you have a pick'n'pull junkyard nearby, you might find one there.
If you have a drill and some drill bits, just drill a hole in the middle of what remains of the petcock. Then wedge something in there - like the nose of some needle nose pliers, although proper bolt-outs would we better - and unscrew. Carefully.