The pilot reported that about 15 minutes after takeoff he smelled something burning and the cabin filled with smoke. The pilot further reported that the engine lost power and that he was forced to maneuver for an emergency landing. After the loss of power, he heard a "clanging noise" from the engine. While landing in a field, the airplane impacted blueberry bushes and a fence, which resulted in substantial damage to the fuselage. During a postaccident examination by the local law enforcement, it was revealed that the oil filler cap was not in place on the oil filler tube. The oil filler cap retaining chain was still attached to the oil cap and the oil filler tube. Upon further examination of the oil cap and the oil filler tube, no evidence of damage was noted. The pilot reported there were no preimpact mechanical failures or malfunctions with the airframe or engine that would have precluded normal operation.