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How does the engine sound? Does it seem to be running correctly, just not developing full power? Or, is there a misfire, or perhaps running too rich. You mention one cylinder appears wet, have you checked the plugs after a hard full speed run? If one plug still appears wet, perhaps the fuel pump is leaking fuel into whichever cylinder it is plumbed into.
What pitch prop are you using? Someone may have changed the prop and installed one with too much pitch which is just bogging the engine down at high RPM. The only other thing I can think of is that crazy “economizer linkage”, the horizontal thin rod that runs from the throttle roller back to the vertical throttle shaft. This feature allows full advance timing before full throttle opening. The adjustment is easy to mess up, and is much easier to set up with two people. Unless the control throttle cable is connected, then you can advance the throttle fully (in gear) and it should stay put so you can adjust the economizer linkage.