If the kit you have is built anything like the original Spickler kit I would say the fuselage is fine as is. You can replace the 3/16 tail pieces with some denser balsa and pin the tail section in place. I would move the firewall back an inch to help with balance but keep in mind that makes installing your fuel tank more difficult. Make a hatch behind the wing for the RX battery. The wings should be OK except on the original kit, there is no dihedral brace. On mine I just glassed the center. On anything faster I would add a brace to the third rib and glass. Here is a picture of mine, built from a 1980 original Spickler kit.
