The NV 90 size pipe or a Macs 90 size muffled pipe would work well too. The difficult part will be the tuning. I would suggest you start off with the header at about 11”. This will no doubt be on the long side. You will want to swap the prop for a 12x11. The Strega is a pretty draggy airplane so a small prop high rpm setup is not going to work well. With the 12x11 your goal RPM is going to be around 13k. The header can be shortened until you get to the point that the engine will jump onto the pipe once in the air and as it unloads. If you adjust the header length old school while taking readings on the ground the pipe will actually hold the RPM down as the pipe will be tuned to a lower rpm then what it will see once airborne. I used this method with a Rossi .61 and was able to get 12,500 on the ground with an 11x11. When it unloaded in the air it was like someone hit the NOS button.