You might wanna try and run it without the muffler? That engine dont need a muffler pressure as it has a fuel pump so the muffler is only there to make it slightly more quiet. I think if you just clamp a 15cm long silicone tubing over the exhaust header to get rid of the exhaust smoke then the engine also gets quiet enough to be acceptable. You didnt say if you got a header for that engine, but if you dont have it should still be considerably cheaper to buy it alone than the muffler set.