If you want to use an OS120AX, look at my MAC muffler setup. This fits the cowling much better than a pitts. I used an OS exhaust extension then sawed it in half to get the muffler in the right place. I flat filed the edge and bought the correct length screws. Check the photos. I have been running that engine in the Venus for 2 years with no problems. I did move the tank back to the CG and installed a Perry pump. This package runs flawlessly and the power is awesome. I replaced that plastic tube in the photo with an aluminum angled tube tha MAC sells to fit that muffler.