Have you tried emailing their customer service dept with these questions? I have had good response from them, when I had been looking for replacement D ring straps, and harness straps ( the harness straps had disappeared for a short while off their website...customer service was nice enough to send me replacements for free, since they weren't listed on the site at the time). I think the harness and buckles remain the same. On my seats, the old harness had smaller loops which were very difficult to get on/off the splitter plate. My replacement ones have larger loops which makes it so much easier. I think they sell 3 different length replacement harnesses, so you may want to measure yours to see the length.
When my never used replacement cover had a rip on it, they were able to send out a new one to me (thankfully, I had a cover that they still had in stock...a lot of the covers they had run out of...I have a SK XTSL). And when I had heard that they had redesigned the headwings on the later model XTSL's (the old style headwings seem to interfere with the harness more), they sent me the new design headwings for free (I have 2 seats). I would just ask them if they have any more 65SL covers in stock. They just might have some that they'd be willing to sell.
I keep a spare set of harness straps with me for puking emergencies. Change out harness & cover, clean the cover (take memory foam out, throw into washing machine on gentle with cold water, then line dry...it's worked extremely well), and wipe down (& air out for a LONG time--like weeks-- the pukey harness). Luckily it's worked for me (&my kids don't get sick in the car often).
