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First thing I would do is have the tank pressure tested to make sure it doesn't leak. Did the previous owner use the internal tank or did they not use it either? If they didn't use it, you may want to have a professional clean the tank out, and assuming it pressure tests out, replace the fuel, vent and outflow lines which if they haven't been replaced, are not rated for ethanol and you should be good. If the tank doesn't pressure test, then replacing is the only option. It's not all that hard to do, there are plenty of threads on the process.