Recently I had an older bike with a 6 speed freewheel Shimano 600 (Ultegra) components. The crankset was 52/39, the freewheel was 12-26. It was a great riding bike. Any good Shimano compatible (130 BCD) 39t chainring would be a good replacement for the 42. Look for a replacement for the rear cluster, something like a 12-26, 13-28, or 14-28. As the others have pointed out, just make sure it's compatible.

Al