I actually have a NIB Bisson Pitts muffler for the G23/G26. I bought it for my G23 but I ended up mounting the G23 inverted on my GP Spacewalker ARF so I didn't use it. I figure I will eventually get a G-26 and use the BCM Pitts with it, but I've got too many gassers in the 25-31cc size already. Have a look around at the various aftermarket mufflers for the G-26. I still think you'd be better off mounting the G26 inverted on the J3 Cub, but if you decide the BCM Pitts is what you need, let me know and I'll give you a good price on it.

FWIW, I'm putting together a Sportsman Aviation Super Decathlon 86" wing right now with a 30cc Homelite engine. After I'm done with it, I'm gonna put together one of the two World Models 1/4 Cubs I have. I'll put on that a 25.4cc Echo weedie engine, which is very similar to the G26. Too many planes, too many engines, too little time.