The pilot's failure to determine the aircraft's airworthiness before flight, resulting in operation of the aircraft in an unairworthy condition and consequent fuel starvation, and his subsequent failure to maintain adequate airspeed in an emergency turn back to the airfield, resulting in a stall. Factors included: an unapproved fuel tank installation; the pilot's lack of recurrent flight training; the pilot's lack of recent experience; and the aircraft's low altitude at the time control was lost.