DURING CRUISE FLT THE PLT REPORTED THAT THE ENG RPM INCREASED WITHOUT MOVEMENT OF ANY COCKPIT CONTROL. HE RETARDED THEN ADVANCED THE THROTTLE BUT THE ENG RPM DID NOT GO HIGHER THAN 1,700. DURING THE EMERGENCY DESCENT, OIL BEGAN COVERING THE WINDSCREEN. HE TURNED TO LAND AT A NEARBY AIRPORT BUT DIVERTED TO A CLOSER ARPT. WHILE ON FINAL APCH TO A RWY, HE REALIZED THAT THE FLT WAS HIGH & FAST & ADDED POWER TO GO AROUND, BUT THE ENG RPM DID NOT INCREASE. HE PERFORMED A 290-DEGREE TURN & LANDED LONG ON AN ADJACENT GRASS RWY. THE AIRPLANE TRAVELLED OFF THE END, THROUGH A FENCE, THEN CAME TO REST IN A DITCH. EXAM OF THE ENG COMPARTMENT REVEALED THAT THE COVER ASSY OF THE PROPELLER GOVERNOR SEPARATED FROM THE BODY. THE COVER IS SECURED TO THE BODY BY A CLAMP. THE CLAMP WAS TIGHTLY SECURED TO THE BODY. THE GOVERNOR HAD BEEN MODIFIED IN 1988 AFTER MANUFACTURE & HAD ACCUMULATED 430 HRS. THE ACFT HAD ACCUMULATED 530 HRS SINCE MANUFACTURE. THE GOVERNOR HAD BEEN REMOVED & REINSTALLED SEVERAL TIMES. THE FLYWEIGHTS WERE NOT LOCATED. NO DETERMINATION COULD BE MADE AS TO THE REASON FOR THE CAP TO SEPARATE FROM THE BODY.