Personnally i will say, better cockpit sole. The one for accessing the engine is badly design always problem with it.
Better drain system on deck, and like John lee said a shoe on the rudder for not catching rope (Talk to Bika about that when their anchor robe snag on my rudder and Nina jump to rescue the situation during the night ! )
For the sail control, a better system for the boom. The one i got from the old contessa give a big shafing problem and not a lot of sail control. Probably lift a little bit the companion way from the cockpit floor. I got problem with that on heavy seas water coming onboard.
Well that bring another question on the rebuilding .... The goal of making a new Co26 if for what exactly ... for offshore sailor, for cuising, for lake, for weekend cruiser .. The way you sail change a lot the boat that you need !