I never knew (til this thread) that the '78 K used the F2-style keepers. That's weird because the 78K doesn't have the F2 larger valves. Guess Honda finally used up the last of their early style keepers and changed everything over. I'm pretty sure a '78K can run any earlier K head without trouble. A F2 cannot unless you also change the cylinder to a K-model.

You might also be able to run a full set of K1-K7 valves/springs/keepers. Going by that CycleX scan the F2 springs are taller. That might be your trouble. With regular K springs and keepers you could probably run regular K valves in your head if you buy the other 5 valves you need. Or just get another head...