WHILE DESCENDING TO CONFIRM HIS POSITION BECAUSE OF ENCOUNTERING RAIN ON A CROSS COUNTRY FLIGHT THE STUDENT PILOT DISCOVERED AN ENGINE RPM LOSS. AFTER ATTEMPTING TO RESTORE POWER SEVERAL TIMES AN EMERGENCY LANDING WAS MADE INTO A PLOWED FIELD DURING WHICH THE NOSE GEAR COLLAPSED AND THE AIRCRAFT NOSED OVER. ENGINE EXAMINATION REVEALED LOW COMPRESSION ON THE NUMBER 2 CYLINDER DUE TO LEAD DEPOSITS FOUND BETWEEN THE EXHAUST VALVE AND THE VALVE SEAT. THE MAGNETOS WERE BENCH TESTED AND FOUND TO HAVE HIGH ACTIVATION SPEEDS AND ERRATIC SPARK THROUGHOUT THEIR OPERATING RANGES. UPON DISASSEMBLY IT WAS FOUND THAT CARBON DEPOSITS AND TRACKING EXISTED ON THE DISTRIBUTOR GEAR SUPPORT PLATE AND ITS BRUSH WAS SLIGHTLY MISALIGNED AND WORN. THE MAGNETOS USED WERE SLICK ELECTRO 4150 AND 4181 WHICH ARE SEALED UNITS REQUIRING NO FIELD MAINTENANCE. PIPER RECOMMENDS THAT THEY BE EXCHANGED AT THE END OF 900 HOURS IN SERVICE.TOTAL TIME IN SERVICE FOR THE SUBJECT MAGNETOS WAS 714 HOURS.