Check for intake leaks by spraying water from a windex bottle around the carb while the bike is idling. I realize this will be a pain, given where the carb is located on the KE, but if it dies while being sprayed, you have a leak.

Also try some plug chops at high RPM's to see the true condition of your mixture at high engine speeds.

Look to see if your in-line fuel filter is the problem. Those things do not flow fuel very well, which is why I don't recommend them. Put a piece of hose in it's place and see if the problem persists.

One more thing...I'm not sure what you consider high RPM, but take a look at your exhaust pipe. See the expansion chamber? Not a high RPM pipe. You may just be asking more of the engine than the pipe will allow it to do. If you want the formulae for building the perfect two stroke pipe, let me know. If anyone on this forum has the ability to follow through with that project, it's you. :)