THE BUILDER/PLT PURCHASED THE BASIC AIRFRAME & ROTOR KIT FROM BENSEN, BUT PURCHASED THE ENG FROM ANOTHER SOURCE. THE ENG WAS NOT MODIFIED IAW BENSEN INSTRUCTIONS. REPORTEDLY, THE PLT WAS ONLY GOING TO CONDUCT TAXI & GROUND RUNS WHEN HE STARTED AT APRX 1100 CDT. HOWEVER, AFTER A NUMBER OF TAXI & HI SPEED RUNS, THE GYROCOPTER WAS OBSERVED TO TAKEOFF AT APRX 1800 CDT. AFTER TAKEOFF, THE PLT TURNED TO A DOWNWIND, THEN A BASE LEG. A WITNESS REPORTED SEEING SOMETHING FALL FROM THE ACFT BEFORE IT DESCENDED FROM HIS VIEW. SUBSEQUENTLY, THE ACFT CRASHED APRX 1500 FT FROM THE APCH END OF THE RWY IN A 50 TO 55 DEG BANK. A 6 GAL FUEL CAN WAS FOUND APRX 90 FT FROM THE ACFT. THE CAN HAD BEEN ATTACHED WITH 2 ELASTIC SHOCK CORDS, WHICH WAS NOT IAW BENSEN INSTRUCTIONS. PAINT & SCRAPE MARKS ON THE ROTOR BLADES & THE FUEL CAN MATCHED ONE ANOTHER. THE PLT WAS NOT CERTIFICATED & NO EVIDENCE WAS FOUND THAT HE HAD PREVIOUS FLT EXPERIENCE. THE WORD 'ULTRALIGHT' WAS FOUND PAINTED ON THE MAST OF THE GYROCOPTER.