If you can't get the tack socket screw out because the head broke off carefully drill a small hole in it and use a screw extractor to remove it. Use caution when drilling to avoid hitting the headstay inside. Unless it has been replaced by a previous owner the screw is a 1/4 x 20 x 3/8" nylon screw. If you have removed the screw and still can't get the tack socket out tie a Prusik knot around the lower extrusion and use your halyard to pull up slightly to remove the pressure on the tack socket, a little penetrating oil may be necessary to free the socket. If you'd like I'll send you my complete drive unit, #1 in the picture above, if you're willing to pay shipping from the east coast.
