GND WITNESSES REPORTED THE ACFT ENGINE BEGAN TO MISFIRE SHORTLY AFTER DEPARTING RY 24 AT OCEANSIDE MUNI ARPT OCEANSIDE, CA. WHEN THE ACFT REACHED THE DEPARTURE END OF THE RY AT AB OUT 100 FT AGL, THE PLT BEGAN A LEFT TURN. PRIOR TO COMPLETING THE TURN, THE ACFT STALLED A CRASHED INTO A BLDG LOCATED ASOUTH & ADFACENT TO RY 24. BOTH OCCUPANTS ABOARD THE ACFT SUSTAINED FATAL INJURIES. NO PERSONS ON THE GROUND WERE INJURED. THE BLDS WAS SUBSTANTIALLY DAMAGED AND ITS CONTENTS WERE DISTROYED. DISASSEMBLY EXAM OF THE ENGINE REVEALED THAT THE NO. 1 CYL SPARK PLUGS WERE NOT FIREING AND THAT THE NO. 3 CYL PISTON COMPRESSION RING WAS BROKEN & THE UPPER SPARK PLUG WAS OIL SOAKED. THE ABSCENCE OF RING PAR- TICLES IN THE OITL FILTER INDICATED THE RING BROKE DURING THE ACCIDENT FLT. SEVERAL CRANKSHAFT & CON ROD BEARINGS SHOWED HIGH TEMP DISTRESS. MOST OF THE OIL PASSAGE HOLES WERE PARTIALLY BLOCKED BY THE BEARINGS. THE CRANKSHAFT WAS PUSHED REARWARD. THE MAINT LOGS REVEAL TWO PRIOR PROP GND CONTACT. THE ENG HAD NOT BEEN OVHLD & HAD ACCUM MORE THAN 1868 HRS.