Well for me I do not like to add weight at all. If it were my plane every servo in the fuselage would be right behind the firewall. Just because the servo tray is near the back of the wing doesnt mean it has to stay there. Move ignition box as far forward as you can, you can even mount the throttle servo right next to the engine and have the pushrod going backwards to the carb to help. Pull pull with kevlar cord is a good choice. Now if still need more weight look at a bigger engine or twin or some sort. This way you can spin a bigger prop at lower rpm for a more scale look.