There are Aluminum etching products on the market to clean up oxidation, but if you get any overspray onto the alternator - kiss it goodby.
I am a big fan of not washing engines for 30+ years. Go ahead, call me names, but I think risks outweigh benefits.
Not a product endorsement, but this may be worthwhile (try first in a small area) if you must use a degreaser.
Product Description
Eimann Fabrik Engine Degreaser is the latest technology in 100% VOC FREE cleaning and degreasing. It uses a state-of-the-art, non-ionic surfactant developed in Sweden to replace the acid, butyl and butyl ethers normally used in engine degreasers. This new surfactant technology allows Eimann Fabrik Engine Degreaser to quickly and safely remove all manner of dirt, grime, oils, and grease from virtually any surface. This cleaner/degreaser is safe for use on painted and clear coated surfaces, plastic, fiberglass, aluminum, vinyl and rubber. It's also safe for your skin (it can actually be used as a hand cleaner!) and poses no threat for vapor inhalation. Engine Degreaser contains no chlorinated solvents or harsh chemicals and is non-corrosive. Used routinely, Engine Degreaser will keep your engine compartment looking bright and clean.