Re: looking foward to my motor arriving!!
My owners manual for the Power King engine rates the engine at 5000 RPM and lists peak power at the same 6000 RPM you found.
I have had my Dax 70 up to 6400 RPM (shown on Sen Dec Fas Tach) BUT something (not sure what and didn't hold it that high to find out!) sounded "rattly".
Hope this helps.