As I understand the story, Honda had a mad scientist tuner named Jerry Griffith turn out the prototype NS750 engine and then HRC turned out a very small number of parts to convert CX500s into NSs- the heads, the cam, the left-side engine cover, etc. I've been told by Gene Romero who ran the RS team that a total of 7 NS bikes were built and that 6 of the frames were C&J single shock, 1 was a dual-shock by a local tuner. I'm having no luck inserting links into this message, but if you goodle -image NS750, you'll see the Bergstrom bike that was in this article, Chris Carter's NS, and the dual shock NS. These bikes are frightfully rare and never come on the market. RS750s are also rare but around 50 of them were built and they do sell now and again. Anyway, it seems that there were 2 fundamental problems with the NS- 1 was that it overheated and the other was that the crankshaft ended up too high in the chassis (dirttrackers are really sensitive to crank location) and the bike wouldn't hook up well enough. Honda fixed both of those problems in the RS and that bike could still win an AMA GNC if you could get the parts to run one. Honda stopped making RS parts cold at the end of 1988 and while most parts are available (there's a brand new RS crank on EBay right now), you just can't get enough parts on a steady basis to make running one a realistic proposition.
And yes, the AMA was long beholden to H/D. They never outlawed watercooling (currently, the 650 Kawasaki Ninja twin, the SV650 Suzuki and the BMWF800 twin are all doing well in the GNCs and are all water cooled), but they did restrict and add weight to the RS until the XRs could finally keep up and there is supposed to be a letter from Soichiro Honda himself, on Honda letterhead, addressed to the AMA telling the AMA to "Go F... Yourselves" when Honda pulled out of GNC at the end of 1988.
I just joined this forum as I'm starting the process of building a CX dirttracker. It will have to be turned 90 degrees like the NS to allow a chain drive and it looks like I might be able to do the head swap to get the carbs on the right (when we fall in dirttrack, 90% of the time it's onto the left side and you'll just rip the carbs off if you leave the layout stock). I haven't decided how to address the crank height issue but have some ideas. If I ask goofy questions, it's because I'm pursuing a goofy project!