Welcome to the board! The USS is a great machine, I've had many throughout the years, including my daily driver in the basement.
This should be the thermostat you are looking for:
http://www.fun-tronicsllc.com/index.php?crn=304&rn=608&action=show_detailIt'll clamp right back in the same place that the original is, my machine cycles between 36 and 45 degrees, for reference.