THE PLT REPORTED THAT HE WAS SPRAYING A FIELD THAT WAS BORDERED BY OBSTACLES. WHEN HE WAS ABOUT HALF WAY THRU A PROCEDURE TURN, THE ENG BEGAN 'BACKFIRING & LOSING POWER.' THE PLT MOVED THE THROTTLE TO ABOUT THE 1/2 POWER SETTING, HOPING THE CONDITION WOULD BE CORRECTED, BUT TO NO AVAIL. THERE WAS A HIGH POWER LINE ON ONE SIDE OF THE FIELD & A 300 FT TRANSMITTING TOWER ON THE OTHER. ALSO, THERE WAS A WOODED AREA AT THE END OF THE FIELD. THEREFORE, THE PLT ELECTED TO MAKE AN EMERGENCY LANDING IN A SMALL, ROLLING FIELD. DURING THE LANDING, THE GEAR COLLAPSED & THE WINGS WERE DAMAGED. AN EXAM OF THE ENG REVEALED THAT THE FRONT SPARK PLUGS WERE SOOTY; THE REAR SPARK PLUGS WERE RUSTED & COULD NOT BE REMOVED; THE #2, #4 & #8 CYLINDERS HAD BLOWN EXHAUST GASKETS; THE LEFT MAGNETO TIMING BUSHING WAS WORN EXCESSIVELY & THE MAGNETO WAS FOUND TO BE LOOSE.