Engines lose about 2 to 3% power per 1000' of altitude. But what is actually important is what is called "Density Altitude". This is true, or actual altitude corrected for temperature and barometric pressure differences, and is a better indicator of air density and consequently the effect it has on engine power.

Higher altitude, higher temperatures, lower barometric pressure, and high humidity all LOWER air density and LOWER engine performance. In all full size aircraft operating manuals, the actual power percentage loss is stated. Due to both the loss of power AND the loss of effectiveness of the propeller and wings due to the thinner air, a take off at about a 7000' density altitude will be twice as long, and the airplane will climb half as fast.

Short of supercharging, the only help is higher static compression ratio, more nitro, or more cubic inches... Even these things will not improve the losses suffered by the prop and wings however.

AV8TOR