Yeah definitely a DIY project for the rear bench. You can buy bunk carpet specifically made for trailer bunks. I would spray the trailer, unless you don't mind seeing brush strokes.
Show us some pics of the engine with the LPG conversion.
Not to steer you away from here, but there's a UK and Europe contingent on the CCFan forums that have LPG conversions and have done a lot of repairs and updates, and upholstery work too.
I'm a Correct Craft owner but appreciate all inboards and there's some great information on this site!
Current boats 1984 Ski Nautique, 1969 Correct Craft San Juan (project)
Previous 1992 Sport Nautique