THE PLT SAID A DSCNT FM 21,000' TO 2500' WAS MADE AT IDLE PWR. HE STATED HE BELIEVES HE HAD PUT THE MIXTURE TO RICH DRG DSCNT. WHILE ADDING PWR TO INTERCEPT THE GLIDESLOPE, THE PROP SOUNDED AS IF IT RAN AWAY. HE CONCLUDED AN ENG FAILURE OCCURRED, PLACED THE ELEC BOOST PUMP ON HI & ATMTD A RESTART. THE ENG FIRED SVRL TIMES, BUT DID NOT START. A FORCED LNDG WAS MADE IN A TREE FARM & THE ACFT NOSED OVER. AFTER THE ACDNT, FUEL STAINS WERE FND ON THE TOPS OF EACH WING NR THE FUEL CAPS; LARGE AREAS OF DIRT UNDER THE WINGS SMELLED OF FUEL. AFTER RCVRY, A FUEL SUPPLY WAS CONNECTED; THEN THE ENG WAS STARTED; IT OPERD WITH NO EVIDENCE OF FAILURE OR MALFUNCTION. THE PROP & PROP GOVERNOR WERE INSPECTED WITH NO ABNORMALITIES FND. THE ACFT HANDBOOK STATED THE MIXTURE SHOULD BE PLACED TO RICH FOR ALL IDLE POWER DSCNTS. HANDBOOK ALSO STATED THAT WHILE RESTARTING THE ENG IN FLT, THE ELEC BOOST PUMP SHOULD BE OPERD ON HI FOR 2-3 SEC, THEN PSND TO LOW. IF LEFT ON HI, THE ENG WOULD FLOOD, WHICH WOULD BE INDCD BY THE ENG FIRING SVRL TIMES, BUT NOT STARTING.