I've had two engines in mine; a Saito 72 four stroke and the current engine which is an OS50SX. I have a 1600ma NIMH battery in the compartment under the fuel tank. With the Saito I originally had a couple of ounces in the nose to keep the Cg toward the front of the range. I eventually removed the weight as I gained confidence in the plane and allowed the Cg to go back near the rear limit. It's more toward the middle of the range now with the heavier 2-stroke and no added weight.

Key for me was putting the battery under the fuel tank.

Greg