WHILE ON A SIGHTSEEING FLT IN THE MT.ST.HELENS NATIONAL VOLCANIC MONUMENT AREA THE ACFT CRASHED ON THE MUD FLATS OF THE NORTH FORK OF THE TOUTLE RIVER IN A RELATIVELY FLAT & OPEN AREA. THERE WERE INDICATIONS OF A MASSIVE OIL LEAK FROM THE ENG AREA IN FLIGHT. ENG EXAMINATION REVEALED A DEMOUNTED NO.3 CYLINDER. HOLD DOWN BOLTS WERE MISSING FROM ONE 1/2 INCH STUD & ONE 1/2 INCH THROUGHBOLT, & FROM TWO 3/8 INCH STUDS. THERE WAS NO EVIDENCE OF TENSILE OVERLOADING OR STRIPPING. ONE HOLD DOWN NUT OF EACH SIZE WAS LOCATED UNATTACHED & CAPTURED IN THE EXTERIOR ENG BAFFLING. THE NO. 3 CONROD CAP WAS MISSING. THE ENG HAD ACCRUED 446 HRS SINCE OVERHAUL & 24 HRS SINCE LAST INSPECTION. THE WRECKAGE WAS FOUND NEAR A LEVEL STRAIGHT ROAD FREE OF SHOULDER OBSTRUCTIONS. A SURVIVING PAX STATED A FORCED LANDING ON A ROAD WOULD BE MADE. A STEEP TURN WAS MADE OVER THE ROAD & THE ACFT "FELL." THE OWNER'S MANUAL STATES THE PLT SHOULD BE AT 1000FT AGL AT THE DOWNWINDPOSITION FOR A NORMAL APPROACH TO A POWER-OFF LANDING. THE STALL WARNING SYSTEM WAS REPORTED INOPERATIVE.