Rule of thumb for naturally aspirated motors of ANY kind, gas, diesel, fuel injected or otherwise is 3% / 1000' of elevation. or at 6600' you lose 20% of your power. Which is why turbo motors are so popular in the Rocky Mtns. They are altitude compensating to 10,000', basically the turbo spools up faster with the thinner air to the set pressure of the waste gate.
Now if they just made a low pressure turbo 600 cc, high torque dual sport that weighed under 400 lbs.