According to the sport pilot, during takeoff in his experimental light sport airplane equipped with a two cylinder engine, he noted a loss of engine power. During the forced off airport landing, the airplane struck a fence post followed by a hard landing. The airplane sustained substantial damage to the horizontal stabilizer and fuselage. A postaccident examination of the engine revealed that the number 2 spark plug had pulled out of the cylinder head and the threads from the cylinder head were still attached to the spark plug.