One choice is as follows:
As Y stated, purchase a simple on off ball valve for caustic fluids (Ace or other hardware stores sometimes carry them), usually 1/8". Wait until tank is low or empty and remove fitting from tank bottom and install brass elbow. Save original inside gas line and original valve as backup or for show. Purchase (extra) stock A inside gas line and measure for the new ball valve with compression fittings on each end and install valve at midpoint or so of stock gas line. Have had no trouble so far with ethanol laced gas.