If it's the breather their are a few ways to fix the problem, let me know and I can send you pictures of my repair.
If it is the breather, your vent filter will be real oily and there will be a nice pool of oil slightly above your starter. As you bob around that can spill over and cause a mess.
Other than that their are lots of places that oil can spill from.
Clean it real well, get your mirror and start looking.
Good luck,
Peter