You no doubt had a Campagnolo derailleur on that bike, running a five-speed cassette, so you'll want the same thing. A Web site called Bicycleclassics.com has Nuovo Record derailleurs from the early eighties that might workcheck out the site and drop them an e-mail to see. Another possibility is a site called Loosescrews.com, which has some older "Campy" stuff as well. Alternatively, you could drag the bike into the 21st century by upgrading to an eight-speed or nine-speed rear cassette. But, that would entail new wheels, shifters, derailleurs, the whole shootin' match, and it's not even a sure bet you could modify the frame to accommodate the newer, wider rear axles.
I'd also suggest you could simply pop for a new bike. But I rather believe you would take a dim view of that suggestion, so I'll refrain from making it.