WHILE CRUISING AT 1600 FT MSL ON TOP OF CLOUDS, A LOSS OF POWER IN THE LEFT ENG OCCURRED. THE PLT ATTEMPTED TO RESTART THE ENG, BUT WAS UNABLE. HE FEATHER THE ENG WHILE DIVERTING TO TONOPATH, NV. THE ACFT WOULD NOT MAINTAIN ALT USING 1 ENG. THE LAST REPORTED WX AT TONOPAH WAS 400 FT BROKEN, 800 FT OVERCAST. OVER TONOPAH, THE PLT MADE A DESCENDING SPIRAL IN IMC. DURING THE DESCENT, ICING CONDITIONS WERE ENCOUNTERED. THE ACFT BROKE OUT OF THE CLOUDS AT ABOUT 400 FT AGL (AT NIGHT), BUT THE PLT COULD NOT SEE THE ARPT. HE STILL COULD NOT MAINTAIN ALT & MADE A FORCED LANDING WITH THE GEAR RETRACTED. AN EXAM OF THE LEFT ENG REVEALED NO PREIMPACT FAILURE OR MALFUNCTION. AN EXAM OF THE LEFT FUEL SELECTOR BOWL REVEALED SIGNS OF MICROBIOLOGICAL GROWTH. ALSO, THERE WAS HEAVY MICROBIOLOGICAL GROWTH ON THE LEFT FUEL FILTER. WATER & MICROBIOLOGICAL GROWTH WERE FOUND IN THE LEFT FUEL TANK. STC SA117NW REQUIRED THE ADDITION OF ANTI-ICING & ANTI-BIOLOGICAL ADDITIVES TO THE FUEL. NO ADDIVTUE WAS USED AT THE PREVIOUS REFUELING.