The factory muffler on that saw is probably just a can and as good as it gets. You can modd it though
You want a REAL UNRESTRICTED muffler? I put this spice can on a parts Husky 55 once. Blew the top off, so I had to add a hose clamp. I'd sell the spice can real cheap.
The rest of the good running, Husky 55 parts saw will cause saw purists to swoon. Cover your eyes! I took redneckitry to a whole new level. The breather is a skoal can and foam rubber filter. The choke rod is a coat hanger. The kill switch is off a computer power supply. The carb is an MS250 carb-used to get for $4 shipped on ebay auction.
61's are great old saws, I've owned a few and still have two that I have converted for more power, very easy saw to make the swap to a 268 or 272 top end, I made one of each and they both see regular use, later model 61's seam to run real good without updating to a closed port cylinder design, I owned a very nice example late 61 and kept it that way.