Most of the mufflers suggested (i. e Macs) may compromise performance on the tower engine, since the original muffler is quite large and gives a special boost. The JustEngines muffler I told you is basically the same volume and shape, but as I said, I´m not sure if they are bolt on or bolt through.

I´m also pretty sure the muffler from the O.S engines or thunder tiger is not a direct fit, since the tower has a slightly different bolt spacing (if memory serves well) and is also bolt through, on the other hand, I think the super tigre muffler for the 75 or 91 fits this and they can be set to be bolt through, since they come with a flange that supports a nut, where the others not.