Not a lot of moving pieces under the 51-55 1st seat? Usually they just need cleaning or new grease. I am not aware of any vendor selling parts for rebuilding them. Clean and lube them, make sure the small adjustment lock spring is intact and the rod is there that connects the two tracks for release. There is a grommet that holds the rod to the seat frame which is available in reproduction.
https://www.flickr.com/photos/advance-design_parts_co/12315510004/in/set-72157640522200463