Aluminum polishing is cheap to do but labor-intensive. You can buy a buffing wheel for your 1/4" drill at any hardware store, which will also probably carry jewellers rouge in a variety of grades. Put down some double-sticky tape to hold the cover in place on a board or bench and start in buffing. The thing will be shiny again in an hour or so. To keep it that way, wax the part occasionally. Good exercise....