he's kidding about the aluminum oxide powder & iron (pretty damn sure it's just a recipe for defective backwards thermite & engine destruction abrasives lol)
not saying you should (still risky, probably illegal, etc.) but a better way would be just drill a little hole so you can insert a sparkler into the exhaust at the very end past the expansion chamber in the last baffle of the carb if it has baffles. That could work on its own (s**t, maybe it would turn out to be a useful way to check for overly rich condition). if not you could have another hole about two inches before the sparkler w/ a narrow tube for injecting liquid butane down it. just make sure you would want to make it so that you only connected it when you shoot out the flames & keep the butane cylinder away normally. If you made it the way I'm saying it would probably shoot fire out the top where you're putting the butane in; solving this issue is left as an exercise to the reader.