If it was my lawn tractor, I would pull the carburetor and check the plastic float for heat damage, and other heat related issues. The fuel pump diaphragm is probably damage also from the fire. Just do it right and replace the fuel lines, and any damage wires that have shown the fire damage. (One wire at a time) There probably no internal engine components damage, due to the fact it is a air cooled engine and it can take a lot of heat other than the seal at the lower crankshaft.