Describe your power problem and the circumstances at which it is running out of power. At the altitudes of the mountains in North Carolina I cannot imagine any problem the stock Mule would have that would be carburetor related unless the carburetor were partially plugged or the fuel filter or air filter were dirty.
If it has been a while since the valves were adjusted that can cut power considerably, as can dirty or old spark plugs. Torque converter clutches that have not been maintained and have a lot of hours can also dramatically reduce power by failing to shift down when they should.