A CESSNA 175 COLLIDED WITH A TREE AFTER A TOTAL LOSS OF POWER DURING A VFR X-COUNTRY FLT. THE PLT INDICATED HE EXPERIENCED AN ENG FAILURE AT 2,000 FT MSL, WHILE ON DESCENT, APRX 15 MI FROM HIS DEST. HE REPORTED THAT THE ENG BEGAN TO VIBRATE SLIGHTLY. THE VIBRATIONS RAPIDLY BECAME WORSE, THEN THE ENG STOPPED SUDDENLY, AND SMOKE ENTERED THE COCKPIT. THE PLT WAS FORCED TO LAND AND SELECTED AN OPEN FIELD BOUNDED BY AN ALMOND ORCHARD. WHILE ON FINAL APCH, THE LEFT WING DIPPED AND STRUCK AN ALMOND TREE CAUSING THE ACFT TO LAND SHORT. THE ACFT CAME TO REST ON THE EDGE OF AN OPEN FIELD WHERE IT NOSED OVER IN A DITCH. THE ENG OIL SYST HAD BEEN MODIFIED WITH PLUMBING THAT ADDED A LINE WHICH EXTENDED THE DRAIN FROM THE DRAIN PLUG TO BELOW THE ENG COWLING. THE LINE WAS PLUGGED AT THE COWLING END AND THERE WAS NO PETCOCK TO RESTRICT THE OIL FROM DRAINING WHEN THE PLUG WAS REMOVED. THERE WAS A LEAK AT ONE OF THE FITTING WELDS WHICH ALLOWED THE ENG OIL TO FLOW OUT. NO OIL WAS FOUND IN THE ENG OIL SYST WHEN IT WAS EXAMINED.