THE PLT STATED THAT APRX 45 MIN AFTER TAKEOFF, HE SMELLED AN ODOR LIKE SOMETHING BURNING,THEN NOTED A LOSS OF POWER. HE STATED THAT THE ENG DID NOT QUIT, BUT HAD LOST POWER TO ABOUT THE 1000 RPM RANGE. ALSO, HE STATED THAT THE OIL PRESSURE DROPPED TO ABOUT 10 PSI. ADDITIONALLY, HE REPORTED THAT HE 'JOCKEYED' THE THROTTLE & IMMEDIATELY APPLIED CARBURETOR HEAT, BUT NEITHER OF THESE HAD ANY EFFECT. SUBSEQUENTLY, THE ACFT ENCOUNTERED A DITCH DURING A FORCED LANDING & NOSED OVER. A TEARDOWN OF THE ENG REVEALED THAT THE LIFTERS FOR THE #3 & #4 INTAKE VALVES WERE CHIPPED, SPALLED & WORN. ALSO, THE RESPECTIVE CAM LOBES WERE ROUNDED & WORN. THE ENG WAS REASSEMBLE WITH THE ORIGINAL PARTS & OPERATED NORMALLY. THE TEMP & DEW POINT WERE 76 & 64 DEG, RESPECTIVELY. ACCORDING TO ICING PROBABILITY CURVES, SERIOUS ICING COULD OCCUR AT GLIDE POWER & VISIBLE ICING COULD OCCUR AT RATED POWER. THE OIL PUMP, OIL FILTER & OIL SENSORS (TEMP & PRESSURE) CHECKED NORMAL. NO MECHANICAL REASON WAS FOUND FOR THE ENG TO LOSE POWER.