A line hooked to a muffler on a gas engine, whether it be for a smoke system or for tank pressure needs to be made of rubber. The rubber will withstand the heat. The various standard lines/hoses for gasoline use won't take the heat, as you have found out. The way the fuel line to the carb on that engine runs almost if not actually touching the cooling fins on the cylinder, you probably should use rubber tubing for the fuel line too. The advertising mentions that they supply special high temp tubing, (probably just rubber) for use with the engine, so this is a known problem.

If the fitting is large enough on the muffler, you could drill it out and extend it by inserting some 1/8" brass or copper tubing. But the extension will need to be brazed with bronze or silver braze, once again to tolerate the heat. Even the so called silver solder you can get at the hardware stores won't hold up.

AV8TOR