While on a local flight the pilot declared an emergency, reporting that he smelled oil and the cockpit was heating up. After radio contact was lost, a search located the airplane crashed into a rocky hill. The airplane struck the hill at high rate of speed in a slight nose and left wing low attitude. An open, grass covered field surrounded the hill. No mechanical malfunctions/failures of the engine or airframe were found. The pilot had been taking medication for several years for headaches which were not reported to the FAA. Eleven months prior to the accident, the pilot had surgery for lung cancer. Three months prior to the accident, the cancer reoccurred in the pilot's brain. The pilot declined radiation therapy and relied on medication therapy. Four days prior to the accident, the pilot reported weakness and lack of energy and was advised to increase his medication. The pilot continued to fly while being treated for his illness.