If you change to a cotterless crank you might find a few other problems, such as the bottom bracket on that Peugeot (??) has French threading. Then your options are pretty limited and often expensive. By the way, is it a Peugeot?

Personally I LOVE cottered cranks and have a number of pairs. And while that crankset you have isn't the best ever, if the chainring is round and true and the teeth aren't worn, I'd get new cotter pins for $4 and enjoy the ride. Email a store like Harris and buy a set of French 9mm (again I'm assuming it's a Peugeot) cotter pins. Here is a link:

http://harriscyclery.net/itemdetails.cfm?ID=1585

Like the page says, it's a good idea to replace both at once. And it might also be a good idea to repack your bottom bracket bearings at the same time as it probably hasn't been done since that bike left the factory 35 years ago. Here's an excellent tutorial on how to do that:

http://www.bikeforums.net/showthread.php?t=208402