The aerial application flight was pulling up from his second tract run of the day, at an altitude of 100 to 150 feet agl, when the engine made a 'clunk' sound and stopped running. The pilot felt his only option, given the low altitude, was a forced landing to a 'wide row' in the citrus grove. Subsequent examination of the engine revealed a crankshaft counterweight retaining pin had migrated from its installed position causing catastrophic internal failure.