THE PILOTS WERE PARTICIPATING IN A GRAND PRIX AIR RACE. AFTER A NORMAL PREFLIGHT, THEY TOOK OFF & CROSSED OVER THE ARPT FOR RACE TIMING PURPOSES. A CLIMB WAS BEGUN ON COURSE, AND A SHORT TIME LATER, THE ENGINE BEGAN LOOSING POWER. THE AIRCREW TURNED BACK TOWARD THE ARPT, BUT THE ENGINE BEGAN TO RUN ROUGH, THEN QUIT. DURING A FORCED LANDING IN A MUDDY FIELD, THE NOSE GEAR COLLAPSED & THE ACFT STOPPED ABRUPTLY. AN EXAM OF THE ENGINE REVEALED THAT A CONNECTING ROD HAD FAILED & PUNCTURED THE CRANKCASE. BEARING DAMAGE WAS EVIDENT THRU-OUT THE ENGINE REFLECTING LACK OF LUBRICATION. TWO MAGNETO DRIVE CUSHIONS WERE FOUND BLOCKING THE OIL SYS PICK-UP TUBE. THESE WERE IN ADDITION TO THE TWO WHICH WERE PROPERLY INSTALLED IN THE MAGNETO DRIVE SYS. AN OPENING BETWEEN THE ACCESSORY SECTION & THE OIL SUMP WOULD ALLOW THE CUSHIONS TO ENTER THE OIL SUMP.