As a reference ,my corsair had a total flying weight of 4.0 kg(about 8lbs) ,it flew ok with the 90 r3 , 13x8 apc prop,but i would not had added much more weight on it. The space. 40 was better for the engine. The other thing with the corsair ,was that you had to fly with full throttle all the time ,and then the sound was`nt that great..
I also opened the cowl with c.flaps to provide cooling.