You need to remove the cover and replace the seal. There is no easy fix i'm afraid. Pull the pulley off, remove the throttle linkage AND the spring inside, don't forget that one or you will stretch it. Replace the gasket, oil pump O rings, the oring on the wear sleeve and the main front seal.
You *may* be able to remove the large pulley and the wear ring that slips over the crank snout, it may give you enough room to pull the seal out, but i can't promise that. Its worth a go first, if it works great, if not, you will need to pull the alloy cover off.
I hope thats of some use.
