THE PLT REPORTED HE 'PICKED UP THE ACFT AFTER ANNUAL INSPECTION', PERFORMED A PREFLT, MADE A STANDARD RUN-UP & TOOK OFF IN CALM AIR CONDITIONS. AFTER CLMBING ABOUT 500 TO 600 FT, THE ENG LOST POWER & SUBSEQUENTLY, THE ACFT CRASH LANDED IN A MARSH. THE PLT WAS HOSPITALIZED FOR ABOUT 2 MOS WITH SERIOUS HEAD INJURIES. THE ACFT WAS NOT EQUIPPED WITH SHOULDERHARNESSES. AN EXAM OF THE ACFT REVEALED NO PREIMPACT PART MALFUNCTION OR FAILURE & THE ENG OPERATED NORMALLY WHEN CHECKED ON A TEST STAND. THE FUEL SHUTOFF VALVE WAS FOUND IN A NEARLY OFF POSITION. TWO OTHER CESSNA 150'S WERE CHECKED TO EVALUATE THEIR ENG PERFORMANCE WITH THEIR SHUTOFF VALVES POSITIONED AS FOUND IN N6901S. THEIR ENGS OPERATED NORMALLY AT IDLE RPM, BUT AT FULL THROTTLE, THE ENGS WOULD LOSE POWER AFTER APRX 21 SEC. THE MECHANIC, WHO PERFORMED THE 100 HR INSPECTION, REMEMBERED CHECKING THE VALVE, BUT DID NOT RECALL THE POSITION THAT HE LEFT IT IN. THE CESSNA SERVICE MANUAL RECOMMENDS THE VALVE BE SAFE TIED TO THE 'ON' POSITION.