I don't have a dohc but on the sohc bikes, the 69-76 K years were all the same pretty much as far as mounting went. The 77/78 were the same too. Now this was model specific.
BUT you could swap and f and k seat but the lines might not look quite right and it may either be a little too long or might leave a gap between the seat and tank. It should fit though for mounting.
The ride IS the adventure. The destination is just to get gas!
Check your locale for motorcycle junk yards. Recently visited one nearby... Tragic to see sooo many bikes, in various states of rust/decay! Parts abounded, though. Had I known of what they had, how close they were, and how low their prices seem to be, I would have never bothered to look on e-Bay for much of what I needed for my bikes.
It is possible to make a custom seat pan, and seat, if necessary. I would not recommend this unless you really cannot find a replacement. If you go this route, make sure you design your seat to fit your desires and expectations. Do a Google search on custom motorcycle seats, and you will find firms that will do the seat building on top of your pan, as well as handing out instructions on how to fabricate a seat pan for your bike, using fiberglass materials. Fabrication of a seat pan is not terribly complex, it is just tedious work. Cheers!
1979 CB750K (sold, 2012, but not forgotten)
1983 Kawasaki 440 LTD Belt Drive (sold, 2011)
1993 Kawasaki Voyager XII