It looks like you are missing a large piece of metal that would hold the seat on. It would be necessary to have a welder fabricate a fix for you. Unfortunately a break will affect the value, but at least this fix would be hidden up under the seat. If it were mine, I would just clean the iron and oil it to prevent rust, if it needs it at all. The iron looks pretty darn good except for the break under the seat. I'm guessing that the wooden seat may need restoration if it is really bad, but I'm for leaving us much of a piece's history as possible. It is hard to tell from the photo, but unless it is really wasted, I'd leave the wooden seat be too, and just get a repair done so that it is functional.