I'm looking at a 1981 750k and I'm concerned about parts availability. In my research, I mostly find tons of aftermarket materials for the older SOHC models and very little for the DOHC. Is there something I'm missing? Why is there so much for one and not the other? If anyone knows of a quality site that caters to the DOHC's, I'd love to hear it. Also, I've heard the interchangeability of these models isn't very much; is that true as well? A few parts I'm concerned about in particular are a new seat and possibly a tank.
A seat can be rebuilt. Tanks are more challenging, but it may be repairable. Other than that, BikeBandit.com sells most OEM parts, though some are wildly over-priced, IMO. I own a '79 CB750K, and I have not had difficulty finding parts for it thus far (since 2006). YMMV, of course.
For difficult to find parts, try creating an automated search on e-Bay. I've found parts that way, including a brand new, still shrink-wrapped, engine guard bar, for a 1983 Kawasaki 440 LTD! You might be amazed at what pops up on e-Bay -- but you have to have an active search running, or you will miss out. Cheers!
1979 CB750K (sold, 2012, but not forgotten)
1983 Kawasaki 440 LTD Belt Drive (sold, 2011)
1993 Kawasaki Voyager XII