Was messing around in the garage today and decided to stick my forks together for a comparison:
SP1/'03-'04 636/5th Gen
On the Kawi forks, the bottom clamp diameter is 52mm and the top is 50mm. SP1 is 54mm and 50mm.
As you can see, the Kawi and SP1 forks are essentially the same length.
The stem on the 636 is too big to fit any bearings into the VFR head, but the stem is pressed into both of the bottom clamps, and both are 35mm. In theory the SP1 stem could be fit into the 636 bottom clamp.
I tired putting the 636 forks in the 636 bottom clamp, and fitting the SP1 top clamp to see if they would go together, but my Kawasaki bits are far too bent up to go together anymore. (That's why they are still sitting around in the garage!) Was close, and I think they may work. I don't have a top clamp from the 636, but I think the center hole is slightly larger.
If I recall correctly, the '05-'06 636 forks had similar measurements, but the stem was shorter and fatter than the earlier. I don't recall if the later Big Piston Forks have the same clamping measurements as the earlier 636 models. Could be worth lurking on a Ninja forum to find out.
All 636 and newer are radial calipers.