You probably have a Morse Rack and pinion steering set up. If you can swing it I'd buy a Simrad AP 12 or AP 14 which comes with a whole new rotary set up. All you have to do is replace the cable. Autopilot is one of the best additions I've made to my boat.
http://www.simradusa.net/index.php?o...d=18&Itemid=39