Try The Works Toilet Bowl Cleaner, a mild hydrochloric acid:
http://www.theworkscleans.com/index.htmlApply with tooth brush and scrub with Scotch Brite while wearing rubber gloves.
To preserve the finish apply Alodine 1001 or 1201, an aircraft coating for aluminum. 1001 is clear (invisible after it dries) and 1201 is the same except tinted slightly yellowish, but not much, simply as an indicator that the solution has been applied. I believe that Mopar aluminum alternator housing castings were coated with 1201 or similar during manufacture, and that Jim Ridge at Dixie Restorations recoats alternator restorations with 1201. I also believe Tremec transmission castings are coated with 1201.
Alodine is a hazardous product containing chromic acid, hydrofluoric acid, and potassium ferricyanide.
