You can change that seal, but chances are it will still leak. I've tried it on a 74 and a 78. Both continued to leak and niether one's shaft was worn..
To fix it take out the phillips head screw, pull the cable housing out of the engine.Leave the old seal in place. Degrease the whole housing , espessially where it contacts the seal and put a bead of RTV silicone around that flat side leaving the opening for the cable drive open completely. Don't get it too thck or you'll have trouble pushing it in all the way later. About 1/16 - 1/8th inch thick is good. Let it dry overnight to create a silicone gasket. Then the next day take a Q-tip and dry all the oil out of the drive hole in the engine cover. Then use a clean one with some type of degreaser like Brake Kleen on it to swab out the hole. Lastly use a dry one to dry the hole up.
Finally, use your finger and smear a thin coating of silicone around the cable housing where it goes in the engine.Get some in the retaining groove too, but not too much. Push it in the engine. You'll have to push a bit harder to compress that ring of silicone you laid around the end of the housing. Then while holding it in put the retaining screw back in, wipe up any silicone that may haqve squeezed out and wait a few hours to let it set up before you run the engine.
I can guarantee you it will never leak again unless you take it out again.