Yes, they did make a muffler. Here is a pic; the style is exactly the same as for the larger engines. It took me a long time to find mufflers for some small Super Tigre engines I had. In the end I found two from a guy in Australia - from a wanted ad on RCG. One is a beautiful polished one in the standard style; the other is a drum or barrel shaped. So, after market mufflers exist in addition to the factory ones. When I was looking, I had a response from Raindave who indicated the K&B 20 (and 28) sportster series mufflers are a perfect fit. These show up on ebay, are available from MECOA, and used from Bob's engines http://bobsengines.com/.

The bolt size for the G20 23 and X15 15/19 are identical. The G15 may also be the same. The 40 size is too big and will not fit.

Jim