What is causing the clunking? If it's the freewheel, flush it out and re-lubricate. If the noise persists you'll have to hunt down a replacement. If it's the hub bearings, open it up and check the bearings and cones. If they're OK, clean it out, repack with fresh grease and bearing and ride on. If the cones are shot, you'll have to find a donor hub for replacements. The biggest problem with Helicomatic hubs was that the bearings were too small and tended to wear out quickly. If you don't catch this soon enough, the cones get ruined as well. The higher end Helicomatics were actually quite nice apart from that design flaw -- lovely ground and polished cones and races as smooth-running as anything from Campagnolo. They just didn't last as long.