GA SMA EXPERIENCED COMPLETE ENGINE FAILURE AND MADE EMERGENCY LNDG WHEELS UP IN PLOWED FIELD.

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CRUISING AT 9500' MY ENGINE SUFFERED SEVERE INTERNAL DAMAGE. AT THE PRESENT TIME IT IS BELIEVED A PISTON PIN BROKE. A SUBSEQUENT INVESTIGATION SHOWED TWO AD'S BOTH 50 HOUR RECURRING HAD NOT BEEN SIGNED OFF 71-09-07 AND 76-07-12. I PURCHASED THE ACFT APPROX TWO WEEKS PREVIOUS TO THIS AND HAD A PRE-PURCHASE INSPECTION DONE ON IT. THE MECHANIC OVERLOOKED THESE TWO AD'S. THIS WAS IN NO WAY RELATED TO THE ENGINE DAMAGE SUSTAINED BUT IN ORDER TO BECOME A MORE SAFETY CONSCIOUS PLT I SHOULD CHECK ALL AD'S MYSELF. CALLBACK CONVERSATION WITH REPORTER REVEALED THE FOLLOWING: CAUSE OF THE ENGINE FAILURE WAS A VALVE FAILURE WHICH FAILED AND KNOCKED A PISTON AND CONSEQUENTLY REST OF ENGINE. FORCED LNDG WAS IN A PLOWED FIELD AND PLT ELECTED TO LAND WHEELS UP TO PREVENT FLIPPING OVER. AS A RESULT ENGINE MOUNTS WERE STRETCHED AND OTHER DAMAGE WAS SUCH THAT REPAIR COST WAS SO MUCH PLANE WAS TOTALED. AD'S THAT HAD NOT BEEN SIGNED OFF HAD NOTHING TO DO WITH ENGINE FAILURE AND NOW THINK WERE DONE BUT JUST NOT SIGNED OFF. FAA INVESTIGATED INCIDENT BUT NO FOLLOW UP ACTION BEING TAKEN.