rhm (Rudi) is a forum member that's a whiz at restoring saddles. I'd suggest using a Scotchbrite pad on the seatpost to start, see where you're at with pitting. Oxcylic acid or equivalent for the rust on the rail clamps.

If the pitting is bad, slow work over a period of time to remove a few mm's of alloy around the top portion of the post. It takes time, but the result is well worth it.

DD