BFR TKOF, THE HELICOPTER WAS SVCD WITH 100LL FUEL (AN APPROVED ALTN FUEL IF THE BOOST PUMP IS USED IN FLT). DRG FLT, THE ENG LOST PWR. AN AUTOROTATION WAS MADE IN TALL VEGETATION, BUT DRG THE LNDG, THE MAIN ROTOR BLADES SEVERED THE TAIL BOOM. AN INSPN OF THE FUEL SYS REVEALED NO EVIDENCE OF CONTAMINATION. THE PNEUMATIC SYS WAS PRESSURE-TESTED & NO LEAKS WERE FND. POST-ACDNT ATMTS TO START THE ENG WERE UNSUCCESSFUL UNTIL THE FUEL SYS WAS PURGED OF AIR. IT THEN STARTED & OPERATED NORMALLY FOR ABOUT 5 MIN, BUT THE FUEL PUMP WAS FOUND TO BE UNSERVICEABLE WITH WORN SEALS & BEARINGS. A BENCH CHECK SHOWED THE FUEL PUMP HAD A FLOW RATE OF 700 PPH AT 600 PSI, THE PUMP SHOULD HAVE HAD A MIN FLOW RATE OF 1269 PPH. ALSO, THE POWER TURBINE GOVERNOR WAS FND TO BE SLIGHTLY OUT OF ADJUSTMENT & A FUEL NOZZLE HAD EXCESSIVE STREAKS & VOIDS IN ITS SPRAY PATTERN. PERSONNEL AT THE MANUFACTURER BELIEVED THE ENG MAY HAVE LOST PWR DUE TO A COMBINATION OF A WORN FUEL PUMP & THE USE OF 100LL FUEL ON A 94 DEG DAY OR FROM VAPOR LOCK.