I own a 1951 Alfa Romeo 6C2500SS which was originally sold to a german
dealer Kosta G.mbH.  Is this dealer in business anymore? I believe he
was in Hamburg.

Hi,

I looked at the German Gelbe Seiten - Yellow Pages and the telephone
book. No entries for a Kosta GmbH that would deal with anything
remotly automotive(at least in the last thirty years it's spelled
without a point after G.). I also checked car dealers in Hamburg and
that did not turn up anything either...
It is quite possible that a company that was active in 1951, is no
longer in business today.