1. You might consider using a bigger battery. If you gotta add weight might as well be useful.
2. Consider adding tail wires. Again, useful weight and that's pretty far back.
3. Use a bigger tail wheel if it's a dragger.
4. Move 1 or more of the servo's to the rear of the plane on the outside with a shorter link.

I would fly it at least once before making radical changes. Make sure you key in lots of elevator movement. Good luck