A bigger muffler will help to a certain extent. An engine is like a vacumm (I'm sure you've heard all this before) The more air you get through it, the better. Now, larger exhaust will help it breathe more, but some backpreasure IS needed. So you don't want a 3in dia exhaust tube, it's doing more harm than good. Now, less bends and curves will help alot, less restrictions. But you'll only get as much air out of an engine as you can get in. So now you have to have an intake with less restrictions, or maybe a scoop of some sort, or low restriction air filter. But when you add more air, you'll have to adjust the fuel/ air mixture. I don't know if all this will effect a 50cc motor or not, but I have a homemade super charger on my 150cc 4 stroke and a 1.5 dia exhaust and it goes when it's turned on! Now, I did adjust the fuel a little to compensate for the extra air, so when it's off it runs a little rich. So yes, a larger exhaust will help, but too large will hurt. However backpressure in 2 strokes are more important than in 4 strokes