Where did the scabbard come from? It looks almost made for the sword, many a dealer would pass it off as so. As to cleaning the best advice is very carefully
what is the stain? is it rust? you cannot get rust on top of gold. Will bits push off using a blunt scraper like a butter knife. You might have to leave some of it on the gilding and just clean around it. To me it looks that it just needs some oil rubbed in with the very finest grade steel wool . Tim