During flight, the engine lost power and the propeller stopped. The passenger recalled hearing a 'ticking' sound from the engine prior to the power loss. According to the pilot and the passenger, the aircraft stalled and began to spin as the pilot tried to turn into a bean field during a forced landing approach. An exam of the engine revealed a hole had been burned through the top of the #4 cylinder near a spark plug hole. The piston tops were free of carbon and had evidence of excessive heat, which would be indicative of detonation. The #1 piston was scuffed, there was a small section missing from the top side of the piston, and there was a hole in the piston at the bottom compression ring. A fuel sample was checked and did not meet the specifications for aviation gasoline, but had characteristics of automotive fuel. Additionally, Champion RHB32E spark plugs were found in the engine, but were not approved in the Continental IO-360-D.