You'd have about 6 inches wide by 10-12 inches long, and about 10 inches high usable space in the cabin, close enough to the c/g that you could add some equipment. The amount of fuel it carries will vary with the size tank, usually 14-18 oz for a 1.50 engine, and the empty weight of the plane (minus fuel, extraneous equipment, but including receiver, battery and servos) would be generally 12-17 pounds. Flight time with the 1.50 and 18 oz of fuel might reach 20 minutes with good throttle management (assuming a 4-stroke engine), but adding weight will reduce flight time; as wing loading goes up, so does speed to maintain flight, which requires higher throttle settings, which decreases fuel economy....etc. The Saito would be my choice for power, or even a 1.80 if you plan on loading the plane down, but neither engine is very stingy on fuel (for a 4-stroke, but still much better than equivalent 2-stroke) If the plane were built light, say 12-14 pounds, I don't think it would be unreasonable to say you could add as much as 5-7 pounds of equipment, so long as it could be arranged in the cabin to maintain a flyable c/g. These are generalities, of course, but you can maybe get a feel for whether you'd be able to do what you want. Even though the cub has a large wing, passing 20 pounds is going to put the wing loading around 35 oz./sq. foot (s.w.a.g.). It won't be a glider.