At about 200 feet agl after take-off, the engine began to vibrate and lose power. The pilot turned the aircraft to the right to position over more suitable terrain. The aircraft was stalled into small trees and bushes before touching down heavily on the nosewheel, which broke off. The aircraft then overturned and came to rest inverted. The power loss was caused by two engine exhaust valves sticking open. Although the valves were found to have been set at the minimum recommended clearance, a build up of combustion residue was present which probably restricted valve movement. Operations in high ambient temperatures involving slow flight and reduced engine cooling at rich mixtures can promote a combustion residue build up reducing valve guide clearance and resulting in valve sticking. The aircraft had been operating in the western Queensland summer conducting sheep survey operations.