If the bore is heavily pitted you need to replace it or have it sleeved.
To remove rust on items like these, degrease and remove the scale from the part, then dunk it in a bucket of vinegar for a while. Check it every so often and hit it with scotchbrite. Vinegar will remove all corrosion and not remove paint. When it is done rinse it completely and coat the exposed surfaces with crisco or "red rubber grease". Nether is petroleum based and can then be cleaned with beake cleaner without fear.
