Tank you very much: advice please

open the fuel line that supplies the carb from the fuel pump. Pull the coil wire and use the fuel pump to pump the gas into a bucket. Rinse and repeat...LOL but really this will allow you to see what's in the bucket. Pump out every bit the pump will pickup. If it's bad the worst rust and sediment will be at the bottom. This will give you a reasonable assessment of what is going on in the tank before dropping the tank and cleaning or replacing. You might luck out. If you are paying someone else replace the tank.