Free is always worth a chance. These mowers are pretty tough, and you should be able to bring it back to life. I don't think the seat is available anymore, but a universal one should work on it. I would definitely take the carb off first, and clean it out, and also change the hydro oil (10w30 for engine and hydro).
If you look down the list of posts, there are parts suppliers listed that you can look up parts drawings and availability. These mowers were made between 1989-1991.
Rattosh51
YT6800