Did you ever get it done? There are several threads on here abou modding mufflers. A drill bit and file works, but if you're not careful you'll set the logs your cutting on fire. The best thing is some stainless pipe at about a 45° away from the forward face of the muffler. This not only minimizes the fire danger, but also reduces the saw's loudness as you don't have the sound waves echoing straight back towards you. IIt also looks pretty slick. It's easier with a 2-piece muffler, but if you have access to a Dremel, cutoff wheel or other cutting device you can open t up to do the mod and then weld it back to together. You might be able to solder it, but I'd weld it for strength.
Or, you could ship it to Mastermind and have him do it for you. He and John do an excellent job!