6400 doesn't seem bad for a 32cc gasser to me, I'm not sure if it could be considered a design flaw, but rather the engine was designed for a different purpose, you could try going up in prop size and see if your RPMs drop as you expect, as an example, I have a 98CC conversion out of a 2 stroke snow thrower, it runs about 6500 RPM, but it will happily swing quite a large propeller, the engine was designed for a higher torque application.

A limiting factor may also be how efficiently the fuel/air mixture gets from the crankcase into the combustion chamber, check the transfer ports for casting ridges and bad angles that will mess with the fuel flow. There are many Youtube tutorials on port tuning and polishing.