Front derailleurs aren't a whole lot better now than they were 40 years ago so you should be able to easily find something that will work. Showing us a picture of the original item will help. Here are the parameters for your search:

1. What's the method of mounting the derailleur? Is it clamped to the downtube or is it screwed into a braze-on (It is probably a clamp on).

2. Is it a top-pull or bottom pull? (It's probably bottom pull).

3. How is the cable routed? Does it come from under the bottom bracket? Does it have a guide above the bottom bracket? Or does the derailleur have a thing that holds the cable sheath on its end and then also clamps the end itself?

For instance, my Schwinn Varisty has a 1 inch tube, the derailluer pulls from the bottom, and it has its own cable stop. On my Fuji, the tube is bigger (I've never measured it), the cable pulls from the bottom and there is a cable guide that is part of the frame so the cable is only attached at one place to the derailleur.

You should be able to take your off, find a local bike shop that handles older bikes, and match it up with one from the bin of parts they've taken off old bikes. Don't worry so much about the brand as with finding one that matches the three items above.