Ok straight pipe vs muffler spark arrestor aside for now, basically you will gain a marginal power increase from removing a muffler. Taking into account that an engine relies on spark, fuel and air. The air component has two thing the amount of oxygen coming in an how fast the waste from the combustion (explosion) can escape and how cleanly (Without restrictions, bends twists chambers etc) without back pressure. Hence why drag cars have straight pipes, turbo prop planes don't have mufflers etc, apart from having to carry the extra weight. So in a nutshell if your machine is tuned for the freer flow then you will by rights make more power. Now being a 56 hp engine you may get an extra 3-4 % improvement, the more you free up air flow the greater the gain. Some say you'll get 5 -10 HP which is a bit of a BS statement as it doesn't take into account what your starting with.
So is the performance gain that noticeable? I'll let you decide that. Stick a muffler on if you want it to be quieter and see if there is a difference, if not at least you won't have to put up with the noise, pissing off the family and neighbors and all that to boot.
Cheers Geo.