LSA gross is 1320#. A 1946 J-3C max gross is 1220# per TCDS. I'm unaware of any weight increase STC other than pre-war Cubs with 1100 and 1170# gross which brings them up to 1220#. All the added lights and wing tank will make empty weight increase but not increase the gross consequently it will still be eligible for operations under LSA rules. Now, it may well have such limited useful load remaining that it cannot carry but one person. Even that is doubtful unless one is really large.