DURING FLIGHT, THE ENGINE BEGAN LOSING POWER AND BOTH OCCUPANTS DETECTED A STRONG ODOR OF GASOLINE. THE PILOT MADE A SUCCESSFUL FORCED LANDING IN A FIELD. HE STATED THAT THE ENGINE LOST ALL POWER ON FINAL APPROACH. AFTER COMING TO A STOP, THE PILOT AND PASSENGER STILL SMELLED GAS FUMES AND EXITED THE AIRCRAFT. AT ABOUT THAT TIME, THEY REALIZED THE PLANE HAD STARTED TO BURN. THE PASSENGER, AND A&P MECHANIC STATED THAT HE SAW BURNING FUEL DRIPPING FROM THE ENGINE COWLING. MOST OF THE AIRCRAFT AND NEARLY ALL OF THE FUEL SYSTEM WAS DEMOLISHED BY FIRE. AN INSPECTION OF THE ENGINE REVEALED THAT THE CRANKSHAFT OIL SEAL WAS OUT OF POSITION AND THE PRIMER FITTING-BOSS WAS BROKEN OFF AT THE RIGHT ELBOW. HOWEVER, INVESTIGATORS WERE UNABLE TO DETERMINE WHEN THE PRIMER FITTING HAD BROKEN. NO PREIMPACT FAILURES OF THE ENGINE WERE VERIFIED.