I use mothers aluminum polish on bare aluminum. If your intending to polish a case first strip it down to the bare aluminum then go through the process of sanding in stages to get it so its polishable. Then use a machine buffing wheel and hit it with a few stages of ruge. then finish off with some mothers polish.
To get it looking like new however this is not the process. Someone else can step in with that one as i have no idea.
You can buy 1 1/2 X 3" scotchbrite wheels to spin in a drill or die grinder or 6" for a bench grinder. This will get rid of any scale or oxy and make the aluminum nice and bright again. If you want show polish start sanding with super fine papers (600 and finer) then on to machine or hand polishing. Depends how flashy you want.
If you are talking about the staining from dirt on a cylinder or head or something, I would try some aluminum wheel cleaner spray and let is sit for a little while, then use a pressure washer to get that off. I haven't ever done this to my motor, but it works well on works finish pipes to get the rust off.