When you build and fly more aerobatics you'll start to feel light is right. A gas engine will fly this plane well but will limit 3D abilities because of weight. IMHO 120 size are too small for gas to get the desirable wing loading, but most people will be happy with it. Instead of adding more weight or a heavier engine, plan while you build and arrange the gear to balance. I didnt need to add weight and I have to admit, I would strip the plane and rework it before adding lead.