I have a Beomaster 5000 and Beogram CD 50 that could use replacement or refurbishment of the aluminum pieces on the top of the units.  Are these aluminum pieces available from any source?  If so, where?  If not, what is the best way to refinish them as original?

Given they're 25 years old, replacements wouldn't be available from B&O.  I've found that metal polish removes some marks but don't rub too hard or else you'll give the metal a shiny finish instead of matte.  If yours are in really bad shape you might consider buying a unit in need of repair (wtih good cosmetics) off of eBay.  Swap the enclosure with yours, then re-sell the broken one.