Re: i think my "80cc" has issues. top speed only 28mph with shifter kit...
I would get a good plug reading and take it from there. If you don't know what's going on inside the combustion chamber, you are just whistling in the dark when it comes to changing things to "make it run better".
28 mph is not a terrible speed for the first few hundred miles.
Some engines break in after a hundred, some seem to take 400...some never do run very fast or quit vibrating, luck of the draw there.
I would go to 24:1 synthetic oil and then read the plug.
20:1 is too rich in oil, making an artificially lean mixture.