From my information, the Saito 100 is a couple of ounces lighter than the Magnum. With a heavier engine, I would have to have the battery in the main fuselage instead of under the tank.

Mounting position for the engine, choice of servos, propeller, spinner, fuel valves, etc could make enough difference to require some nose weight but I wouldn't imagine that it could be very much for the Magnum 91.

Ross