I have a Platt Zero at 18 pounds that is fast with a OS 160fx at sealevel. People have flown this Zero with the recommended OS 108 but I am not that good of a pilot and a bit of extra power is not bad. You could adust the power/speed with a different prop also. I am not sure of the lose of power at your altitude but I think I would go with the 210 Mark or one from Dave Patrick that can use some Nitro.