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 two thirds and 8.5 degrees the rest) 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 is good, it's at a fair price ($98), and it's available at California Motorbikes (left pipe). Next best is the pipe on the right for $53 from Amazon. The "step" at the header/diffuser cone connectin will reduce pipe effectiveness.
Concerning these pipes, their headers need to be longer to not have a really high peak RPM of the pipe (which you can't acheive if the porting isn't radically changed). For the $53 pipe, if 22.25" (565mm) length for this pipe (till the beginning of the contracting cone) is correct and the exhaust port duration is 155 degrees then that equates to the maximum allowable RPM of this pipe to be 11,000 RPM! Most engines only rev to 7000. For a 7K engine with 155 duration the pipe (till baffle) needs to be 35 inches (900mm) which is 12.75" (335mm) longer than the present 22.25". You could do some cutting and welding to increase the header length for the max RPM to match your porting.
Here's the $53 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.
Here's a stock silencer sawed off of a stock pipe and welded on:
Even better, but costing you some effort, is my pipe which I designed for these engines which you can make from printable patterns for cutting out sheet metal for each pipe section that will need to be rolled. The design includes a silencer welded on and without needing occassional service. To make an upswept header just 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 and 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.
The guy that made this one pictured here said it was "dramatcally better than the MZ65 clone pipe" he had tried before. This doesn't surprise me at all. Of course a pipe designed for a specific application will be better than all others. Of course.