THE PLT OF A QUICKIE Q-2 HAD RETURNED TO THE ARPT & MADE 2 LOW PASSES, THEN BEGAN A NORMAL APCH TO RWY 25. WHEN THE Q-2 WAS ON A LOW FINAL, THE PLT OF A MOONEY ACFT TURNED ONTO THE RWY & BEGAN TAKING OFF. AT ABOUT THAT TIME, A GROUND WITNESS HEARD THE FOLLOWING TRANSMISSION OVER THE UNICOM FREQ, 'THANKS A LOT MR. MOONEY, NEXT TIME YOU DO THAT I'LL CHOP YOUR TAIL OFF.' THE Q-2 WAS THEN OBSERVED TO MAKE A VERY ABRUPT LEFT TURN TO REENTER DOWNWIND - TO-BASE. THE Q-2 PLT WAS THEN STATED (ON UNICOM) THAT THE ENG WAS SPUTTERING & QUITTING & THAT HE WAS 'GOING IN.' DURING AN ATTEMPT TO LAND ON A CONSTRUCTION BLUFF, THE ACFT COLLIDED WITH LARGE ROCKS, THEN BURNED. NO PREIMPACT STRUCTURAL FAILURE WAS FOUND & AN ENG TEARDOWN REVEALED NO INTERNAL PROBLEMS. HOWEVER, THERE WERE SEVERAL ENG MODIFICATIONS THAT HAD BEEN MADE BY OTHER THAN FACTORY REPS. THE THROTTLE CABLE BRACKET WAS MADE OF NON-STANDARD MATERIAL & WAS DEFORMED BY HEAT & WEAR. A THROTTLE CABLE CLAMP WAS MISSING. THROTTLE FULL ON, BUT CARB HOUSING CLOSED.