Howdy TX and welcome to the forum! Your seat replacement is 81268001 (Mirage backrest, expanded peg). If you find the seat pad a bit harsh, I recommend the i-comfort pad (72020028) (I use it on all my Hobie seats). You'll have to check to see if the straps come with the seat or are ordered separately.
Your older rudder is not convertible to Twist and Stow, but it sounds like it just needs a tune-up. When you crank the rudder down, check for any slack on the down line at the rudder and take out any slack. Also look for slack on your left and right rudder lines (at the rudder) -- they should be taut, but not tight. That's actually a good little rudder -- just doesn't flip up like the T&S. Parts are ordered through a local dealer. Have a look at the online Kayak and Fishing Parts and Accessories catalog for replacement parts (near the back) and accessories.
The forum is a gold mine for info and learning how to use the search feature is the key to extracting it! For instance, with model year, go to the oldest posts first when looking up boat-related matters. Nevertheless, if you have other questions don't hesitate to ask!