its been a few years since i converted mine but you need this.
http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXTXK0&P=7
i think you also need a new start shaft but i can't remember. I snapped my start shaft so i converted at the same time. You get the rotostart backplate and the S25 pullstart bolts right on. It may be more expensive now but unless they changed the design, that OS pullstart will fail again soon.
http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXTXK0&P=7
i think you also need a new start shaft but i can't remember. I snapped my start shaft so i converted at the same time. You get the rotostart backplate and the S25 pullstart bolts right on. It may be more expensive now but unless they changed the design, that OS pullstart will fail again soon.