THE PILOT OF THE MULTI-ENGINE AIRCRAFT REPORTED THAT HIS RIGHT ENGINE WAS LOSING OIL AS HE WAS CLIMBING TO CRUISE ALTITUDE. HE SECURED THE ENGINE, FEATHERED THE PROPELLER, AND MANEUVERED TO RETURN TO THE AIRPORT. THE PILOT TRIMMED THE AIRCRAFT FOR SINGLE ENGINE OPERATION AND EXTENDED THE MAIN LANDING GEAR. DURING THE APPROACH, THE OPERATING LEFT ENGINE WOULD NOT DEVELOP FULL POWER AND THE PILOT REPORTED TO ATC THAT HE COULD NOT MAKE THE AIRPORT. THE AIRCRAFT CONTINUED ITS EMERGENCY DESCENT, IMPACTED TREES, AND DESCENDED UNCONTROLLED THROUGH A DENSELY WOODED AREA. A FUEL TANK RUPTURED DURING THE UNCONTROLLED DESCENT THROUGH THE TREES AND A FIRE/EXPLOSION OCCURRED. THE AIRCRAFT CONTINUED TO BURN AFTER GROUND IMPACT. BOTH ENGINES WERE DISASSEMBLED AND NUMEROUS MECHANICAL ANOMALIES WERE FOUND. THEY HAD BEEN RECENTLY OVERHAULED. THIS WAS THE FIRST FLIGHT OF THE AIRCRAFT SINCE THE ENGINE OVERHAULS. THE PILOT/OWNER WAS EN ROUTE TO SELL THE AIRCRAFT WHEN THE ACCIDENT OCCURRED.