AFTER TAKEOFF, THE PILOT WAS CLEARED TO PROCEED ON AN IFR FLIGHT PLAN. AFTER BEING CLEARED TO 15,000 FT MSL, HE REPORTED HAVING TROUBLE MAINTAINING HIS AIRSPEED AND SUBSEQUENTLY DECLARED AN EMERGENCY. SHORTLY AFTER THAT, RADAR CONTACT WAS LOST. THE AIRCRAFT IMPACTED AT NIGHT NEAR THE TOP OF A MOUNTAIN RIDGE AT ABOUT 4700 FT MSL. THE WRECKAGE WAS DEMOLISHED AND SCATTERED OVER A LARGE AREA, BUT THERE WAS NO EVIDENCE OF AN INFLIGHT BREAKUP. SOME PARTS OF THE WRECKAGE (INCLUDING THE RIGHT ENGINE) WERE NOT FOUND ON THE SNOW COVERED TERRAIN. THE WEATHER FORECAST CALLED FOR: THUNDERSTORMS IN LINES, MOUNTAINS OBSCURED, MODERATE AND OCCASIONALLY SEVERE TURBULENCE BELOW 20,000 FT, OCCASIONAL MODERATE ICING IN CLOUDS AND IN PRECIPITATION FROM THE FREEZING LEVEL (3000 FT) TO 20,000 FT. THE AIRCRAFT WAS NOT CERTIFIED FOR FLIGHT IN KNOWN ICING CONDITIONS.