Once you get past the headers, and into the collector, you no longer have to deal with the "tuned" length. A resonator is just a straight through glass pack muffler. There should be almost no exhaust pressure added when going through one. Another possibility is a chamber pipe like the Corvette side pipes. Some early planes with inline engines ran a long exhaust pipe with a muffler, or at least an expansion chamber. I have seen pics of them with it going along the right side of the fuselage.
As an added note, most of the noise you hear when flying is the propeller. There was an experiment where a prop was affixed to an electric motor, and the same prop was affixed to a piston engine. The difference in overall volume was about 10% at the same RPM. If you can find a quiet prop, then I would worry about a muffler.
You might even consider electronic noise attenuation. Some of the new trucks have it, and it eliminates the exhaust sound by inducing a 180 degree out of phase signal into the exhaust pipe with a transducer. That would not eliminate the prop noise though.
The noise from the prop is caused by the tips approaching the speed of sound, and making a shock wave. Simitar props are supposed to be quieter than a straight blade prop. That may or may not be true, as I have not checked personally. The props with the small "winglet" air dam on the end may be quieter as well. They are more efficient, and you can run a smaller prop at the same RPM, with the same thrust, so the tips do not get as close to the speed of sound.
The Russian "Bear" bomber ran its props so fast that they substantially broke the sound barrier at the tips. It was so loud, that someone flying next to them in another plane would be extremely uncomfortable because of the volume of the prop noise.