The pilot reported that the aircraft engine experienced a loss of power while being vectored for the initial ILS approach to Charleston, SC (CHS). Attempts by the pilot to regain power failed and a forced landing was made in a swampy field a few miles north of CHS. The aircraft nosed over inverted as it came to a stop and remained that way until it was removed from the site 3 days later. Examination of the aircraft revealed about 6 to 8 ounces of fuel in the fuel system and no contamination was detected. A total of 4 gallons of fuel was then added to the two wing tanks. The engine was readily started, using the aircraft battery, and it ran smoothly after a few seconds of roughness. The engine was run to full power and no defects were noted. Following shutdown, an examination of the fuel system revealed no leaks. The pilot stated that the aircraft was topped off prior to departure and should have had 5 hours of fuel. The pilot reported the total engine running time at 4 hours 40 minutes.