This is an interesting discussion, covering many issues.
Before turnaround I flew MK Arrows with OS 61VF and an OPS pipe set conservative, somewhere between 13,500 and 14,000 RPM on the ground. The airplane weighed about 7.5 pounds and went like stink. The field where I flew had a noise problem and I needed to get quiet. I added a soft mount, went to a 12-9 prop and lengthened the pipe about 2”. I dropped 10 DB and gained performance burning about half the fuel and the glow plug lasted all season. I was especially impressed with the authority with which it pulled away from the corners on the square loop with half rolls.
My first turnaround airplane used an OS 61 SF and Mac quiet pipe turning a 13-9 APC, which was really pushing it. I later converted to an OS 91-FS turning the same prop almost exactly the same RPM’s. The power system weight was about equal but the four-stroke weight was all up front requiring lead in the tail.
You don’t necessarily need a four cycle to keep the noise down. Davis Diesel Development http://davisdieseldevelopment.com/muffler/index.htm has a variety of Sound Master mufflers and quiet pipes. I have a remote mount SM-GMF, which as I remember somewhat tuned with a header extension and was noticeably quiet. It has been a long time.
Jim