THE PRIVATE PILOT ENCOUNTERED FREEZING FOG AND HAZE, REDUUCING THE VISIBILITY TO THREE MILES. HE TURNED BACK TO LAND BUT THE ENGINE BEGAN TO RUN ROUGH. HE ADDED CARBURETOR HEAT BUT THE ENGINE BECAME ROUGHER AND REDUCED TO PARTIAL POWER. THE PILOT SELECTED A RIDGE IN ROUGH TERRAIN AND MADE A FORCED LANDING. AT TOUCHDOWN, THE TAIL LIFTED AND THE PLANE NOSED OVER. THE VERTICAL STABILIZER, ELEVATOR, WINGS, COWLING AND FIREWALL WERE BENT. DURING A POSTACCIDENT ENGINE RUN THE ENGINE OPERATED NORMALLY AND RAN FOR FIVE MINUTES.