THE PILOT STATED THAT DURING THE INITIAL CLIMB AFTER TAKEOFF, HE HEARD A 'POP' & THE ENGINE STARTED RUNNING ROUGH. HE TURNED BACK TOWARD THE AIRPORT. ABOUT 2 MIN LATER, THERE WAS A LOUD NOISE, WHICH WAS FOLLOWED BY A PROGRESSIVE LOSS OF ALL POWER. SUBSEQUENTLY, THE ACFT WAS DAMAGED DURING AN EMERGENCY LANDING AT NIGHT IN A MARSHY AREA ABOUT 1 TO 2 MI FROM THE AIRPORT. AN EXAM OF THE ENGINE REVEALED THE #5 CONNECTING ROD CAP BOLT HAD FAILED, WHICH RESULTED IN FURTHER DAMAGE OF THE ENGINE.