The airplane sustained an in-flight fire and during the forced landing impacted the terrain nose low. According to ATC, the pilot diverted to an alternate airport after reporting smoke in the cockpit. According to witnesses, the pilot did not shut down the engine. Witnesses reported that the airplane entered a downwind pattern and impacted the terrain after overshooting the runway during the approach. Investigation revealed that the fuel line to the No. 5 cylinder had separated over time by cyclic stresses that propagated a fatigue crack. The maintenance installation of the line had been improperly routed and the line not clamped. Manufacturer information on the line routing and clamps had not been available in the service bulletins. The exhaust pipe exhibited mechanical abrasion and thermal oxidation prior to the accident although the exhaust system had undergone a recent maintenance check. A 'B' nut missing on the top spark plug for the No. 5 cylinder had been situated at a location of intense heat during the in-flight fire. Surface cracks penetrating the inside surface of the V-band coupling securing the exhaust pipe exhibited surface oxidation damage determined to be in existence for some time prior to the accident.