You can obtain one of these from www.sewingpartsonline.com, not sure if you mean the cam stack gear or the hook pinon gear down under the rotary hook but both are available, liks are http://www.sewingpartsonline.com/cam-drive-gear-elna-416340.aspx

or http://www.sewingpartsonline.com/hook-drive-gear-elna-star-11-sewing-machine.aspx

If you want to install yourself you may want to join the Yahoo group Elna Heirlooms, they share lots of information there on these Swiss made Elnas.