Best way to clean a stock muffler without cutting or mods is remove the cap & spray it thoroughly with brake cleaner.
.035 plug gap is a bit wide, I run 32 & my bike likes it. If reworking the jets don't help, try different gap settings. This also changes the timing: A smaller gap has less resistance so the charging capacitor in the CDI can achieve avalanch, or cascade effect sooner, releasing the volts to spark. We're only talking a couple milliseconds, but at 6000 RPMs or more, that can mean a couple of degrees. Too much gap can cause the capacitor to hold its charge too long, overheating it & lead to componant failure. An upgraded aftermarket CDI may help out too, some allow you to alter the timing curve with a jumper pin, so you can tune it to what your motor likes.
If jetting changes, plug gaps ang a free flowing exhaust don't fulfill your need for speed, there's always porting. Lots of China Girls get here with excessive casting flash in the transfer ports, litterally choking flow through the engine. This isn't a simple 1 day bolt on proposition, its major surgery. But then again, these little engines aren't complicated, after all, we are talking about 19th century technology. Research the topic using the google search bar at the top of the page. You would be surprised what 20 minutes with a Dremil can accomplish.
One more thing, is your air filter choking you? Run it around the block one time without it & see if your top speed improves.
