The CO danger surely comes from either a badly placed exhaust (which is rare as it then shouldn't be certified / permitted) or through the cabin heating system via a crack in the exhaust, with the exhaust fumes getting into the muffler.
If the plane has a water cooled engine then the cabin heating can come off a water pipe, rather than the exhaust, and so the danger is much reduced. Therefore no real need for a CO detector.
What sort of planes have you been flying?