EARLIER IN THE DAY, AN ANNUAL INSPECTION HAD BEEN COMPLETED. THERE WERE NO REFUELING FACILITIES ON THE ARPT, SO THE PLT/MECHANIC DROVE TO ANOTHER LOCATION & PURCHASED A CAN OF GASOLINE (6 GALS) WHICH HE ADDED TO THE TANKS. HE THEN STARTED THE ENG & SATISFACTORILY COMPLETED A TEST FLT. AT ABOUT 1425 EST, A POTENTIAL BUYER WAS BOARDED & A DEMONSTRATION FLT WAS BEGUN. DURING THE INITIAL CLIMB, AT ABOUT 300 FT AGL, THE ENG BEGAN SPUTTERING. THE PLT TRIED TO RETURN TO THE RWY, BUT THE ENG LOST ALL POWER. DURING A FORCED LANDING, THE ACFT COLLIDED WITH A FENCE. THE FUEL SYS WAS CHECKED & APPEARED TO OPERATE NORMALLY. NOTHING WAS FOUND IN THE GASCOLATOR OR FUEL SCREEN TO BLOCK THE FUEL FLOW. THE PLT SUSPECTED THAT ICE MAY HAVE FORMED IN A LOW POINT IN THE FUEL PLUMBING THAT COULD NOT BE DRAINED. HOWEVER, THE TEMP ROSE ABOVE FREEZING ABOUT 2.4 HRS BEFORE THE FLT. WATER WAS DRAINED FROM A T-FITTING THAT ACTED AS A SUMP, BUT COULD ONLY BE DRAINED BY REMOVING A PLUG.