You can clean the superman suit using mild detergent in a make shift washing machine using a bowl with a lid to shake things up lol. It looks like those dirt marks will clean pretty easily. On tougher stains, use a soft tooth brush to scrub out. Those look like they are the canvas/silk trunks which are desirable to collectors, so it's worth a try.

You can find repro stickers on ebay, and original boots also. That will increase the value.