Shouldn't need any modifications. The vehicle would just be low on horsepower. O2 sensors would sense a rich condition and exu would dial back injector pulse width. At high altitudes, turbos are the only way to go, as a supercharged engine would suffer the same fate as a naturally aspirated, as impeller speed is directly linked to crank speed. With a turbo, as long as it's sized correctly, it will make whatever boost regardless of atmospheric pressure. Just might have some extra lag as the impeller will need to spin faster.