Take the cap off the dizzy and see if there's oil inside it or the main housing. If there is it's the internal oil seal, otherwise it's the "O" ring where the dizzy fits into the head.(easy, cheap to fix) If it's the dizzy's internal seal, Honda don't sell it as a seperate part. New dizzy I'm afraid, or at least a new housing. I have replaced one on an earlier import GA2 City with a seal from a Mazda 323 but you need a specialised bearing puller etc to get to it.