the issue with such a small engine doing this is the huge difference in pressure when the engine blows vs. when its in-taking, at low rpm is will almost suck inwards on the exhaust. this will cause the flammable gas moving through the pipe to jump forward, then step back. this is a recipe for blowing the flame out, a glow plug will always be hot and be able to repeatedly ignite the gas in the pipe regardless of it's flow-state.
this will get you lit, the problem now become that it can be lit going the wrong way, this shouldn't be able to do any damage to your engine, and if your pipe it sturdy enough it will only cause a backfiring effect at low rpm. but it will be possible using a setup like this to have your flamethrower suddenly become a bomb. you can mitigate this by having the flammable gas enter the pipe relatively close to the end, but there needs to be a little space between the gas outlet and the glow plug (in theory the ideal place for the glow plug would be opposite the gas outlet and about an inch further towards the exit of the exhaust.)
this is why the exhaust doing the throwing needs to be customized to this purpose, that said it only needs to be narrow and solid, nothing too fancy.
if you can get a pipe that is fluted at the outlet, but plain tube for the rest of the run, without welded joints along the pipe anywhere but the flange, that would work perfectly.
for reference, the standard way of making these work for movies is to use dummy exhausts that are purpose built flamethrowers that work independent of the vehicle, using a small fan to push air through the pipe into a setup like the one I am describing. using a fan like that eliminates having to build around the air moving power of the engine, and ensures oxygen is inside the pipe before the propane is. because you are using engine exhaust to move the flammable gasses out of the pipe you have to rely on the oxygen outside the pipe mixing with your flammable gas/ exhaust mix as it leaves your pipe (hence why a fluted pipe would be best) if you try and mix the flammable gas with oxygen as it enters your exhaust you will either blowback into that tube making it not work, or fill your exhaust with burning gas moving the wrong way, this will blow up your motor.