PORTION OF THE AIRCRAFT EXHAUST SYSTEM SEPARATED RESULTING IN AN IN-FLIGHT FIRE. THE RESULTANT ENGINE FAILURE NECESSITATED A FORCED LANDING. PILOT ATTEMPTED TO LAND IN A SOFT FIELD. AIRCRAFT FLIPPED INVERTED AND INCURRED SUBSTANTIAL DAMAGE. PILOT WAS NOT INJURED. EXAMINATION OF THE ENGINE EXHAUST SYSTEM REVEALED AN IMPROPER WELD BETWEEN THE MUFFLER AND THE EXHAUST. THE WELD FAILED AND THE MUFFLER SEPARATED.