AFTER EXPERIENCING A PARTIAL LOSS OF ENGINE POWER IN FLIGHT, THE INSTRUCTOR TOOK CONTROL OF THE AIRCRAFT. HE APPLIED CARBURETOR HEAT, AND HEADED BACK TOWARD THE AIRPORT. THERE WAS A GRADUAL LOSS OF POWER AND THE INSTRUCTOR ELECTED TO LAND IN AN OPEN FIELD. THE FIELD WAS SMALL, AND IN PLACES, IT HAD MUD UP TO ONE FOOT DEEP. DURING THE LANDING, THE AIRCRAFT FLIPPED OVER AFTER ROLLING ABOUT 75 TO 100 FT. AN EXAMINATION OF THE ENGINE REVEALED THAT THE NUMBER ONE EXHAUST VALVE ADJUSTING SCREW NUT HAD COME OFF AND JAMMED THE INTAKE VALVE SPRING RESULTING IN A BENT INTAKE VALVE PUSH ROD. ALSO, THE EXHAUST VALVE ADJUSTING SCREW WAS 1 1/4 TURNS FROM COMPLETELY BACKING OFF THE ROCKER ARM. WHEN CHECKED, THE EXHAUST VALVE WOULD NOT OPEN. THE INTAKE VALVE WOULD BARELY OPEN DUE TO THE LARGE CLEARANCE PROVIDED BY THE BENT PUSH ROD.