BEFORE THE FLT, THE PLT WAS OBSERVED PERFORMING MAINTENANCE IN THE ENG COMPARTMENT. HE WAS KNOWN TO DO SOME OF HIS OWN MAINTENANCE WHICH INCLUDED STOPPING AN OIL GASKET LEAK BY USING A SEALER. WHILE CLIMBING AFTER TAKEOFF, HE TURNED BACK TO THE DEPARTURE ARPT, THEN DIVERT TOWARD THE BWI ARPT WITH AN ENG PROBLEM. THE ENG LOST POWER & HE TRIED TO LAND ON A SCHOOL YARD, BUT THE ACFT CRASHED & BURNED BEFORE REACHING IT. AN EXAM REVEALED FATIGUE CRACKS & A HOLE IN THE #4 CYLINDER. POLISHING OF A DISPLACED PIECE OF THE CYLINDER & DEPOSITS IN THE CRACKS SHOWED THAT THEY HAD BEEN THERE FOR AN UNKNOWN PERIOD OF TIME. FOAM PADS (TO KEEP BIRDS OUT OF THE ENG COMPARTMENT WHILE PARKED) WERE FOUND IN THE AIR/VENT HOLES OF THE COWLING. AN ENG TEARDOWN REVEALED: #2 FUEL NOZZLE PARTIALLY OBSTRUCTED; LEFT MAGNETO IMPROPERLY ADJUSTED; RUBBER MATERIAL RESEMBLING SILICONE SEALER WAS FOUND IN THE OIL SYSTEM, BUT NO OIL SYS BLOCKAGE WAS NOTED. EVIDENCE OF HEAT IN ENG BEARINGS JOURNALS. R FUEL CAP NOT VENTED. PLT'S MED CERT EXPIRED. HIGH TIME ENG. PROP STRIKE, 4/9/83.