AT 0939 EDT, THE ACFT DEPARTED SYRACUSE, NY, ON A FLT TO NASHUA, NH. SUBSEQUENTLY, IT WAS RPRTD MISSING. A SEARCH WAS INITIATED, BUT THE ACFT WAS NOT FOUND UNTIL IT WAS DISCOVERED BY A HIKER APRX 2 MONTHS LATER. THE ACFT HAD CRASHED BELOW A MTN TOP NEAR PETERBOROUGH, NH. INITIAL IMPACT WAS WITH TREES ON RISING TRRN AT AN ELEV OF APRX 2000'. THE ACFTCAME TO REST AFTER HITTING SVRL TREES & TRAVELING ABOUT 100' FROM THE INITIAL IMPACT POINT. NO PREIMPACT MECHANICAL MALFUNCTION OR FAILURE OF THE ACFT WAS RPRTD. AN ODOR OF FUEL WAS NOTED, THOUGH THE ACFT HAD REMAINED INVERTED FOR APRX2 MONTHS. DURING A PERIOD FROM 0751 TO 0951 EDT AT MANCHESTER, NH, THE CEILING WENT FROM AN ESTD 5000' BKN, 15,000' BKNTO 1000' OVC. DURING THE SAME PERIOD AT CONCORD, NH, THE CEILING WENT FROM 4000' BKN, 8000' BKN TO 800' BKN, 1900' OVC;AT 1014 EDT, THE CEILING AT CONCORD WAS 1100' OVC; THEN AT 1052, IT INCREASED TO 1700' AT 1052