Hello; When I first built my GB Extra, I put a Magnum 65 SE in it, lots of power but too thirsty for fuel. I then put an OS 91 FS in it, that worked pretty good, but I (mistakenly) listened to the other fellows at the field and put a Saito 120 in it. This flew it with lots of authority but made it that little bit heavier, and took away some of the floatability that I liked with the 91.

I fly at sea level here and can't imagine what it is like to fly at 8000 feet altitude, I think you'll be okay with most any 120 or with a Saito 100. I know that it is the popular thing to over-power planes, but I preferred mine with the 91.