APPROXIMATELY 45 MIN AFTER TAKEOFF, THE ENG STARTED TO RUN ROUGH. THE PLT HEADED TOWARD THE NEAREST ARPT WITH THE ENG STILL RUNNING ROUGH. THE PLT ENTERED DOWNWIND & PROCEEDED TO FOLLOW A NORMAL LANDING PATTERN. AS HE TURNED FINAL, THE ENG QUIT COMPLETELY. THE ACFT HIT TREE TOPS & CAME TO REST IN A SHALLOW CREEK. THE WINGS WERE NOT IN THE WATER. PRIOR TOREMOVING THE ACFT, APPROXIMATELY 1/2 TO 1 OZ OF WATER WAS DRAINED FROM EACH WING. ENG EXAMINATION REVEALED THAT THE TOP PISTON RING FROM CYLINDERS 2 & 4 WERE BROKEN. THE PLT HAD RECEIVED HIS PRIVATE CERTIFICATE 7/82. HE STATED THAT HE HAD NOT PRACTICED A PRECAUTIONARY APPROACH SINCE HIS STUDENT DAYS, & EVEN THOSE WERE DONE TO A MINIMUM ALTITUDE OF SEVERAL HUNDRED FEET. HE ALSO STATED THAT HE HAD NOT BEEN INSTRUCTED ON PRECAUTIONARY APPROACH PROCEDURES WITH ROUGH RUNNING ENGINES OR WITH PARTIAL POWER LOSS.