If that leak is the only issue you want to take care of, pull the balancer and see if it has a groove worn in it. If that is the case, for about $5 you can get a sleeve that fits the balancer to eliminate the leak, or you could replace the balancer. If there is no groove, look closely at the seal. If the rubber of the seal looks ok, perhaps it was installed in the cover with no sealer which would also be an easy fix.