glow,

I have the Aresti 40 with an O.S. 50SX up front and mine balances just aft of the forward CG location. Instead of adding additional weight to the tail, have you tried moving the battery pack as far back as possible? I also replaced the factory tail wheel with a Sullivan unit and I also added tail braces. Also try to mount the engine as far back as possible on the mount. And as a last resort, move the battery pack as far as necessary toward the tail to achieve the desired balance.

I can fly knife edge the entire length of our field, although it has a lot of roll/pitch coupling. Inverted flight on mine barely requires any down elevator to hold. I routinely fly inverted only about 2-3 feet off the deck and I have no problem going vertical at the end of the maneuver.

Hope theis helps. Good luck and happy flying.