WHILE RETURNING TO THE ARPT & TOWING A BANNER, SMOKE ENTERED THE COCKPIT & A SMALL FIRE BEGAN BURING, FORWARD OF THE LEFT/LOWER PART OF THE FIREWALL. THE PLT TURNED THE ACFT TO LAND IN A FIELD. HE OPENED THE RIGHT CABING DOOR, ALL AIR VENTS & THE WINDOW ON THE LEFT SIDE, BUT THE FIRE & SMOKE BECAME WORSE. HE RELEASED THE BANNER IN AN OPEN FIELD & SLIPPED THE ACFT TO IMPROVE HIS ABILITY TO SEE DURING THE LANDING. AFTER TOUCHDOWN, THE NOSE GEAR FAILED & THE ACFT NOSED OVER & BURNED. THE PLT EXITED THE ACFT WITHOUT INJURIES, BUT WAS TREATED FOR SMOKE INHALATION. EVIDENCE OF AN INFLT FIRE WAS FOUND WHERE THE FUEL LINE WAS ROUTED IN THE AREA JUST FORWARD OF THE FIREWALL & THE PLT'S LEFT RUDDER PEDAL. THE FUEL STRAINER & SOME FUEL LINE CONNECTORS WERE LOCATED IN THIS AREA. THEY WERE NOT BURNED, BUT THEY HAD BEEN EXPOSED TO HEAT.