DURING SPRAYING OPERATIONS THE HELICOPTER LOST ENGINE POWER AND THE PILOT ENTERED AN AUTOROTATION. DURING THE DESCENT THE PILOT BECAME AWARE OF A PILE OF LOGS IN HIS PATH SO HE EXTENDED THE GLIDE. TOUCHDOWN WAS MADE TO THE ONLY SPACE AVAILABLE AND THE AIRCRAFT SANK INTO SOFT/WET MUD AND ROLLED ONTO ITS SIDE. AN EXAMINATION OF THE AIRCRAFT REVEALED THE LOSS OF ENGINE POWER WAS DUE TO A FAILED PISTON ROD END RETAINING BOLT.