I think you may need to keep an eye out for the right tank. I've seen them on eBay from time to time. You may be able to find one of the steel tanks from the kdx200sr which seem to be cheaper than the plastic ones. The later model tanks won't fit, as the tank sits 'in' the frame. I'm not sure about kx tanks though.
My bike was also missing the kips valves when I got it, and had next to no power. On a bike that old you should remove the cylinder at the very least to check the plating and inspect/clean/replace the kips parts.
They really awaken with some good reeds, a good pipe (or the pipe mod) and good jetting.
Good luck! :)