Hola Eduardo, we had a similar problem when I moved to Colorado. My regular 40 size airplanes were underpowered and I noticed that most of the guys in the club went up to the next even size engine. If your model is very light and you have marginal flight performance with the current engine first try by increasing the pitch of the propeller. In my case since my airplane was heavily loaded I replace the .46 with a .61 and it flew exactly as it did at sea level conditions. I would recommend the following engine changes:

.10 ---> .20
.20 ---> .40
.40 ---> .60

A little equation that you could use to figure out good approximation for the engine is:

Hp,s = Hp,a((rho, a)/(rho, s))^1/2

where:
Hp,s = horsepower at sea level
Hp,a = horsepower at altitude
rho,s = standard air density at sea level
rho,a = standard air density at that altitude

espero esto te ayude un poco