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take a junker screw driver ,
heat it with a torch,
press into the remaining plug till it melts a slot,
remove and let cool,
then insert the tool and unscrew the plug.
NOTE buy 2 drain plugs and 2 bleed ports for the top of the radiator.
screw the new plugs till snug,
use a smear of Dow Corning 111 on the O rings. keep the extra parts as spares
Follow up. Warmed up an old Philips head screw driver with a lighter for about 40sec and then pushed it into what was left of the plug firmly for 15sec.
Left an almost "factory" impression of the Philips driver and easily was removed.