When I acquired my used tiller the slip clutch was rusted to the point that it would not slip with bolts loosened. Once I chiseled it apart I simply put it back together with bolts loosened and ran it for a few seconds letting the clutch slip. Then tightened the bolts to spec. The slipping will remove surface rust. A few pits are not going to stop it from working. The important thing is that you check it from time to time to ensure it is working properly.
That's a good idea, I've already removed the rust, this sounds like a good thing to do in the future just to make sure it's working.