There are many ways to clean aluminium, one is beadblasting, but that will leave a surface that will quite easily get impregnated with
oil etc, another and you may laugh is "Rhubarb" I have a friend who use to boil old carbs etc in a pot with several Rhubarb leaves, not sure about the stems, rhubarb being mildly acidic is great for this!
Food, meat processing plants use from what I remember an industrial strength orange liquid for cleaning fat etc off alloy and stainless steel sheets, I used on some McLaren F5000 magnesium wheels, and it smoked a bit but did an extremely good job, so you can go for a mild acidic vegetable to the hard stuff!
Also a mag wheel cleaning business should be able to recommend a good product to get the valve covers back to the original look!
Cobrabill may be on to a good product, as I think the industrial cleaners have the hydrochloric acid