AN ENG COMPARTMENT FIRE OCCURRED SHORTLY AFTER TAKEOFF & SMOKE ENTERED THE COCKPIT. THE PLT THEN MADE A FORCED LANDING IN A SAND PIT APRX 1 MI FM THE ARPT. HE & THE PAX EXITED THE ACFT WITH SERIOUS BURNS. AN INV REVEALED AN EXHAUST PIPE HAD FAILED AT A PRE-EXISTING CRACK NEAR FUEL SUPPLY LINES. THE EXHAUST PIPE HAD BEEN PREVIOUSLY WELDED, BUT THE FAILURE WAS UNRELATED TO THE WELD REPAIR. THERE WAS EVIDENCE THE FUEL LINES WERE BURNED BY ESCAPING EXHAUST GASES WHICH RESULTEDIN THE FIRE. DRG AN ANNUAL INSPN ON 12/21/82, EXHAUST SYS CRACKS WERE WELDED & THE MECHANIC RETURNED THE ACFT TO SERVICEAS AIRWORTHY, BUT RECOMMENDED THE EXHAUST SYS BE REPLACED. NO CRACKS WERE FND DRG A SUBSEQUENT ANNUAL INSPN ON 2/2/84 AT A TOTAL AIRFRAME TIME OF 3316 HRS & AN ENG TIME OF 1754 HRS. THE FLT TIME FROM THE LAST INSPN TO THE TIME OF THE ACDNT WAS NOT DETERMINED.