The pilot reported that, approaching destination, he had just reduced power to commence descent from 9000 feet when a rattling noise started. The engine then started to vibrate and run roughly and would only deliver partial power. The aircraft entered cloud at approximately 3500 feet and broke out of the cloud base at about 2600 feet. By this time there was not enough power to maintain level flight so the pilot elected to land uphill in a paddock. The pilot reported that he was a little high on approach and after touchdown the aircraft ran 150-200 metres before going through two fences and hitting a tree. The elevation of the accident site was reported as approximately 1000 feet. The pilot said the cloud cover below the aircraft at the time of the engine malfunction was between seven and eight eighths. Inspection of the engine after the accident revealed a failed connecting rod big end. Significant Factors The following factors were considered relevant to the development of the accident: 1. Failed connecting rod big end in the engine. 2. When the pilot broke out of the cloud base he was only 1600 feet above the terrain and therefore limited in his choice of areas suitable for a forced landing. 3. Forced landing site not suitable to permit a successful (damage free)forced landing.