Spray K2R in the fuel tank area. Let it set up and then blow it out with a air hose. That should fix the fuselage. Spray clear dope in if you can't get your hands in to fuel proof the tank area. The plane is not ruined. People do not through away models because the fuel tank leaked.
I have two 4 Star 60 kits, not arfs, and there is enough room to get a paint brush in to fuel proof the compartment. Glue an extension onto a cheap epoxy glue brush and use either clear dope or Balsa Rite. Put it on the brush heavy with the nose of the plane facing down. If it runs so what. This should solve your problem.