DURING CRUISE FLIGHT THE PILOT HEARD A BANG, FOLLOWED BY A LOSS OF ENGINE POWER. THE AIRPLANE WAS BEING FLOWN OVER DENSELY WOODED, MOUNTAINOUS TERRAIN. A FORCED LANDING WAS MADE TO A LOGGED CLEARING, WHICH RESULTED IN A NOSE OVER WHEN THE NOSE WHEEL CONTACTED A CUT LOG. A SUBSEQUENT EXAMINATION OF THE ENGINE REVEALED THAT THE NUMBER 2 CYLINDER EXHAUST VALVE HAD BECOME STUCK IN THE OPEN POSITION. THE CYLINDER & PISTON EXHIBITED EXTREME IMPACT DAMAGE FROM THE FRACTURED VALVE.