The nose is short and the rear fuselage is long. A small amount of extra weight in the tail requires a large amount of weight in the nose. Did you add a heavy tail wheel assembly with springs and brackets, etc? Lightening stuff in the tail will make a big difference.
The three servos in the fuselage probably weigh around nine ounces and are probably already somewhat near the cg. Moving them to the design cg point will help to some extent. Moving the battery pack to just in back of the firewall will help. A larger heavy battery pack would be better than lead ballast. A heavy prop spinner would help too. A heavier engine would help too.
I once read about a PT17 that lost a prop blade and then the entire radial engine vibrated off. The pilot survived by giving it fill down and landing nearby on a golf course.
