THE ACFT HAD BEEN ABANDONED IN THE BAHAMAS FOR APRX 7 MONTHS. A PLT, WITH NO KNOWN FLT TIME IN A CESSNA 421, FLEW IT TO FT LAUDERDALE. EXCEPT TO AVOID CLDS, HE MADE THE FLT AT A LOW ALT & AIRSPEED. WHEN ASKED WHY, HE SAID HE WAS TRYING TO SAVE THE ENGS. THE PLT THEN DEPD TWD AN AREA OF DARK CLDS & TSTMS. WHEN HE DID NOT ARRIVE AT THE DESTN, A SEARCH WAS INITIATED. THE ACFT WAS FND 4 DAYS LATER WHERE IT CRASHED IN A STEEP NOSE DOWN ATTITUDE. RADAR DATA SHOWED THE ACFT MADESVRL HDG CHGS & WAS RTRNG TO THE DEP ARPT BEFORE IT CRASHED. THE FWD PART OF THE ACFT WAS BURIED IN A SWAMP, BUT THE AFT EDGES OF THE WINGS, FUSELAGE & EMPENNAGE WERE VISIBLE ABOVE WATER. THE PROPS HAD ROTATIONAL DMG; NO PREIMPACT MECH FAILURE OR MALFUNCTION WAS EVIDENT. AN EXAM OF THE RGT ENG REVEALED ITS #2 PUSH RODS & ROCKER ARMS/SHAFTS/RETAINERS HAD BEEN REMOVED BEFORE FLT. ALSO, A #2 SPARK PLUG WAS STOWED WITH ITS IGNITION LEAD ATTACHED. THE ACFT OWNER WAS NOT FND. THE PLT'S MED CERT WAS DATED 5/28/85.