The solo commercial pilot departed from a remote airport on an air taxi/cargo flight. After takeoff, during cruise climb, the engine began to run rough and lose power. Emergency engine procedures did not remedy the roughness and power loss. During the subsequent forced landing in a marshy, muskeg-covered area, the airplane sustained substantial damage to both wings, engine firewall, and fuselage. After recovery, an FAA airworthiness inspector's examination of the engine did not find any mechanical malfunction.