That has to be the question most asked. Many have tried but noise is the nature of our
2-strokes. That's why you can hear the dirt bikers from miles away. Go with a nice quiet 4-stroke if you are trying to fly under the radar.
Fill the muffler with steel pot scourers, it will reduce performance and noise. Then fit rubber blocks in the cylinder fins and glue rubber sheeting or similar to the side covers.