Jak that is looking pretty good. I have to give it to you for persistence, and welding skills. The long center section, the portion that just carries the wave is designed for a slow revving engine, I know you knew that. The first cone amplifies the sound wave the last cone drives it back and shoves the fuel that escaped the port back, acting like a supercharger. The way you tune a pipe is to calculate the speed of sound, and then time it to return to the port at the speed you need it. That looks like a 5hp Briggs muffler, I don't know if that will work it has a block 1/2 way through, all the stingers we used to put on were straight and the length has an affect on the tune of the pipe also. Good work keep it up. Have fun, Dave
PS: If I were guessing at the size of the stinger you need, I would say 1/2 id. and 6" long to start with. You can use a two stroke muffler that emulates those dimensions.