I don't know that guarana would do anything for respiratory problems. Its a concentrated source of xanthines, which tend to be stimulatory. Examples are caffeine, theobromine, and theophylline (all found in cocoa, also). Some vendors try to hide the caffeine, calling it guaranine instead, but its caffeine.

There may well be other active principles in guarana (I'd be surprised if there weren't), but I can't find any evidence for treating congestion.

Lar