I don't think the valve compressors with the little hooks work that well. You need to get the kind that looks like a C-clamp. Or you could just use a C-Clamp with an O2 sensor socket. Both are available at Harbor Freight. The O2 sensor socket has a cut in the side that you can get the keepers in through. If you use a C-clamp, you need a big one to clear both the socket and the head. I would guess that would be about 8-10".
Here is what the official Kawasaki Valve tool looks like:
