once you get your carb tuned and get a better fuel mix your gonna see improvement if all else is right.
without exception all the engines I have run from day one to several hundred miles basically run the top speed they will run withing less than 20 miles, maybe because of how hard I run mine initially I get my rings seated very quick, but I never have seen more that one or maybe two mph increase in top speed between the first day I start the engine until it has even a few hundred miles on it, in my experience the rings seat very quick with these engines so I personally have no doubt that your rings are probably sealed about as good as they ever will be with 125 on it.
Something Tom mentioned here is very true when he said that some of these engines just run better than others, I do believe that 9 times out of 10 all that needs to be done to an underperformer is just change the cylinder jug and clean the ports up good since port timing is one of the biggest factors in whether or not one of these engines are gonna be a good performer or not when all else is correct, if fuel mix is right, timing is correct, compression is good and port timing is where it needs to be, the engine will run good as long as it isnt one of the horribly balanced engines that has a wobbly crank that tries to shake itself to death when it revs a little above idle, but Ive got over 40mph out of even some engines that would feel and sound like they were about to grenade any moment from horrible vibrations so even though that is a limiting factor when all else is right even the badly balanced one can be got to spin up fairly high rpm.
Just get that carb tuned and fuel mix corrected first and lets see what happens.
One more thing I just thought of also, which type exhaust gasket are you using?
A lot of these engines come with the one that just has a round hole and others have a larger rectangular opening, I ask because the stock exhaust is such a limiting factor with these engines and any improvement you can make to help get the exhaust out easier is a big help, I have stocktype pipes on two of my engines but I have ground the flange leading into the header tube so that it is as closely matched to the port as possible and then I use the gaskets that have the rectanguler opening and I match the opening perfectly to the exhaust port in the jug itself to minimize restriction as much as I can so exhaust coming from engine doesn't slam into any flat ares but is directed as good as possible into the header pipe, doing this makes a difference you can feel and see with power and speed compared to how the same engine will run with the same exhaust without anything done to it.
Every little thing you can do to one of these engines to get them breathing better is an improvement that can normally be realized either with increased top speed or power in the mid to upper rpms for going up moderate inclines, these little china gals need all the help they can get.
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