you mentioned it is chopped and bobbed. If you eliminated stock air intakes or exhaust you may need to re-jet carbs if they are still original and may be one source of popping. What is the mileage and have the valve tappets ever been adjusted? Sounds like you have multiple issues... rust in tank, popping, loss of power when warm, loud tapping noise. You may have to thoroughly go thru each system (fuel delivery, ignition electrical, engine mechanical) and make sure they are all ok and serviced.