Relative density is probably about 75% at that height. If you don't wanna use nitro to restore power, then using a larger engine is a good choice. You'll need at least 30% larger displacement. The .51 seems about right. As for the prop, go up in pitch, don't change the diameter. Pitch doesn't affect rpm all that much and it even if it turns slower static rpm, it will turn faster once it's up to speed. A 12 x 7 should work well enough.