I really don't know the answer to your question. Have you considered possibly a motorcycle muffler? something from a single or two cylinder bike? If from a multi cylinder bike, find one that basically has an exhaust pipe/muffler per cylinder.
Might work better than a car muffler do to the motor design. Just a thought I had while thinking over your question. -- You don't quit playing 'cause you're OLD. You're old BECAUSE you quit playing!!
I used to have a V 4 air cooled Wisconsin converted so 2 of the cylinders were an air compressor. Based on the 30 HP Wisconsin engine.
Yes it was loud, cranking out 140 psi.
One car muffler did not do the job. 2 in series kind of kept it down. I was lucky to have a Stainless Steel flexible joint so the manifold did not take too much load of all that exhaust piping. 4 years of use and a hole burn through the SS flex pipe. Air cooled engines do get hot, particularly when run hard.
Perhaps run the exhaust into a metal 5 gallon bucket with some glass fiber inside, and then out. Perhaps chill the exhaust in the bucket to cool the gasses, they do that with marine engines. Let the gas bubble up in the water. That may cause problems in the winter! Getting the weight off exhaust may be tough. Harder if the engine is rubber mounted.
You may not like any of the above but just some different ideas.
Have done this (using small car muffler) on RV Onan generator.. They don't work well for small engines as the cfm capacity of what they were designed for doesn't lend to effective use on a small engine..
They sound like the engine is ummm farting in an oversize jug..
As mentioned in the post above... you must use flexible piping to keep the very significant small engine vibrations from trying to rattle and shake the larger muffer and piping..That noise alone will be significant and puts tremendous stress on the small engine exhaust port...
I bought a length of stainless steel flex piping that outlasted everything.
A good rv generator muffler from a RV store such as Camping World would be very effective, appropriate, but not cheap solution.
Super Trapp is a good suggestion but that's the wrong muffler for the application. Jack's lists 2 Super Trapps designed specifically for small-engines. Forget the car muffler. -- You Can't Kill Rock 'n' Roll!
Something like that might work, but seeing as that's a non-OEM muffler listed on ebay there. I'd doubt it would be any better than the typical pancake muffler. A genuine honda muffler might work. -- You Can't Kill Rock 'n' Roll!
Glass pack mufflers (cherry bomb, etc) are fairly effective for this sort of thing. Keep the muffler close to the engine, and a fairly long run of pipe after it. If you knew the dominant frequency that you are trying to dampen, you could design a resonator to go before the muffler to further the effectiveness of it. There are many calculators out there for designing resonators.
You should consider using a very short run of black pipe to come from the exhaust port to your flex pipe because it will handle the heat. Once you have the exhaust figured out, run a fairly long run of 3 inch PVC for the intake. Just having that will reduce the noise from that end, and shouldn't starve it for air. If the intake is still too loud, a glass pack on that end will reduce the noise to barely audible.