3 years is a good rule of thumb for NiCd batteries. Send the ones you have to a battery recycler, not the trash can.
Chances are the o rings are bad in your engine. I replace mine every year, so there's no doubt you'll need new ones.
You should definitely replumb the tank. The soft parts do harden over time and cause leaks.
To verify your radio, put the antenna down and walk 30 paces. Have someone check that there is no unwanted servo movement, first with the engine off and then with it running (to see if vibration causes problems)