My full length Hooker Headers on my '65 Fairlane were Ceramic Coated by HP Coatings. HP Coatings sells a ceramic header cleaner kit, which should work for all ceramic header coatings, not just theirs.
My powdercoating friend tells me he has had success with such an idea by using high end metal polish. I never tried it but I don't expect he'd tell me stories for no reason. Another venue to try, at the least