Yup, converting to the OEM outside filler is a difficult and expensive task.
in addition to the outside “gill” and access door, one will also have to source a new gas tank outlet, hose, gas cap outlet and the correct left hand engine cover piece.
Perhaps most difficult is the stamped sheet metal piece that attaches to the Gil area and provides an overflow hose exit.
Mike Mayberry has advertised that sheet metal piece in the past and it alone is in the neighborhood of $250, IIRC.
not an impossible task, but just finding all the pieces is a challenge, not to mention their cost.
Larry