If you keep your stock pipe then you should cut down the exit pipe inside the muffler for less restriction and more power. This picture shows where to cut it.
If you want the best power then you definitely need an expansion chamber. For a test with my reed valved 48cc engine I climbed an inclined street (3 degrees the first half and 8.5 degrees the second half) easily with my expansion chamber I designed for these engines and then tried the same with a standard exhaust pipe and couldn't go more than half way up the 3 degree section. Then I took the standard pipe off and tried to blow through it and could feel lots of flow restriction. It's amazing to what lengths the factory went to in order to prevent people from attaining much speed on these things. So then I removed the pipes end piece and cut off 6.5" of the exit pipe and that allowed me to climb all of the 3 degree street but none of the 8.5 degree street. [see video]
So the good news is that you can greatly improve your ride by installing an expansion chamber, but the bad news is that most all of them available that are cheap aren't very good.
Here's my top pick of all the pipes available because its performance isn't bad, its cheap ($59), and it's available at Amazon and other places. If I bought one I would cut off the abrupt transition in diameters at the header/diffuser junction and weld in a cone to smoothen out that transition to prevent loss of wave efficiency.
Here's the same pipe on two different bikes with mufflers attached (which is necessary because it's so loud). The one on the left had the original stinger cut down. The stingers outer diameter is 22mm. The silencer on the left is a $10 Briggs and Stratton muffler which can be found at Home Depot or Amazon and is connected with 1 inch ID heater hose. The silencer on the right is $23 from Amazon.
Even better, but 3x the cost, is my pipe which I designed for these engines. It comes with silencer welded on. If i'm not too busy then you can have me make one for $200 delivered to your door (USA address). It comes without header so you need to do like I did and use two headers off standard exhausts and weld the two curves together. Below are the recommended port durations with corresponding header lengths (centerline starting at piston face). On my street ride I like the 175/117 combo. The pipe as shown below has a 430mm header length. Click here for more info.
This picture shows how this 70mm wide belly pipe isn't dangerous to your leg when the pipe is touching the frame.