use a about 9 inches of bicycle chain with small line attached. turn sideways and push over top of sheave, then turn 90 degrees so chain drops. let gravity be your friend then someone can snag with a hook (bent coat hanger works fine) through the exit plate on the mast. Tie on to new halyard and pull it through. Best if you have a reaving splice at the end of the halyard to attach the chase line.