The aircraft was being operated on a VFR flight and was cruising at 1500 feet above ground level. The engine suddenly began to run roughly and the rpm dropped to about half the normal setting. Emergency trouble checks failed to alleviate the problem and the pilot transmitted a Mayday call as he tracked towards a nearby strip for a precautionary landing. The approach to this strip was too high and power was applied in an attempt to go around. However, severe engine vibration occurred and the pilot was committed to a forced landing in densely forested, hilly terrain. The power loss and vibration was caused by partial separation of the exhaust valve head on one of the cylinders.