Next up polishing all the fins on the cylinders.... any recommendations??
Hydrofluoric (not hydrochloric) acid, industrial strength purchased from a chemical supply place.
Hydrofluoric acid is the "active" ingredient in most mag wheel cleaners or alloy cleaners. As it is seriously nasty stuff, commercial product using it (such as mag wheel cleaner) is usually way too weak to remove road grime and oxidation on engine cases and fins.
If you do buy a high concentration of this stuff, please read the Materials Handling Instructions,and believe them. If you get this on your skin it will do much the same job as the Alien's saliva in the various "Aliens" movies (it is also the stuff used by hitmen to dispose of inconvenient bodies). Go slow, go careful and use a section of your yard where all you will kill is grass, do not use over concrete, because guess what, it eats that too.
If you are lucky enough to live in a country with slightly less strict dangerous goods laws than Australia, you can try for a product by a German firm by the name of Henkel - they have two "paired" products - "Alu-prep" which takes alloy surfaces back to a clean bright finish and "Alu-dine" which is available in various "colours" and is used to provide a long lasting semi impervious finish. (If you have ATE calipers, either on an R65 or the earlier swinging caliper type on bigger BMWs, those calipers were finished OEM with this product, so you can see how damm good it is). Sadly in Australia the dangerous goods laws are such that I can only buy the Henkel products in 20 litre drums, combined with freight from Melbourne (about 3,000km South of me) it is a $600 spend.
Phosphoric acid (usually used for rust control on Steel) will remove oxidation from alloy in quite weak concentrations, but it leaves a very dark, flat and lifeless finish. Have a look at the final drive unit of my R65 as an example.