Summerwind,

From the forward cowl face to the forward surface of the firewall is about 3 1/8" and the diameter is 8 1/2". If you decide to use 4-stroke power you'll probably need to make a clearance box in the firewall.

One thing other builders have made clear is that the N11 is easy to build tail heavy because of the very short distance between firewall and center of lift. I guess all the rotary-engined planes are sensitive like this but the N11 is especially so. So, the heavier weight of your .90FS would probably replace lead anyway. Might as well balance the plane with something useful.

The OS SF series engines however are probably my favorite 2-strokes because of their power, reliability and longevity. If it were me, I'd probably go with the SF and stack my servos up against the inside of the firewall. I plan to place my battery and fuel tank inside the cowl too.