The airplane collided with trees while the pilot was returning to land after encountering a rough running engine shortly after takeoff from the Seminole Lake Gliderport at about 1440 Eastern time. Visual conditions prevailed for maintenance ferry flight Kissimmee, Fla. The airplane was substantially damaged although the Private pilot, the sole occupant, was not injured. The pilot later stated that an engine problem resulted in an uneventful precautionary landing the day before. He replaced the damaged No. 2 cylinder exhaust pushrod, pushrod housing, pushrod seals and lock tab but did not inspect the valve. Following the repairs, the engine was started and a full static run-up was performed with no discrepancies. Approximately two miles after takeoff, the engine started running rough. The pilot elected to return and, after recognizing he was going to overshoot the runway, elected to perform a 360-degree turn on final approach. During the turn, the aircraft impacted trees and came to rest in a swamp.