Perhaps I can fill in a little bit more info about your K-100. It was designed for making coaching films for football. That's why the viewfinders are custom. The viewfinder provides you a view of what's in-frame, and what's out of frame, which is EVTREMELY useful for football filming. In my experience, viewfinders for 25mm and wider are easy to find. Viewfinders for 50 and 75mm lenses are a bit more rare. Additionally you will almost certainly pay more for the viewfinder than the c-mount lens to go with it. Also, the K100 is probably the fastest and easiest camera to load that you wil ever find. It's designed so you can reload between plays.
Regarding the frame rate, if you make yourself a loop of exactly 48 frames, you can use a stopwatch to check the film speed. Since the speed adjustment is continuously variable (no stops) you can adjust to exactly 24 fps by timing your loop. If you find that the camera comes up to speed more slowly than desired, the clutch is adjustable. The spring is good for about 40 seconds of shooting at 24 fps. You might notice the frame rate drops once you get toward the end of the spring though.
The Pan Cinor 17-70 zoom with split viewfinder was also used commonly in football films. I've seen many people use it with the c-clamp trick referenced above. You will find that you will lose about 1/3 of an f-stop because of the split viewfinder.
Good luck. The K-100 is a wonderful piece of gear, and it's virtually indestructable.
