Too bad, I hate when things go “boing.” I wonder how that happened. It’s possible that either the derailleur hanger failed, or the wheel was somehow a little crooked in its dropouts and snagged the derailleur. So the first step is to eliminate any possible causes to this mishap that might still be lurking on the bike. It’s also possible, of course, that the derailleur just fell apart. After all, it WAS 35 years oldB
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 work聴check 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.