I'm going to say you need to try an exhaust first.
More air in needs to let more air out.
Yamaha PW 50 dirt bikes come with a washer installed into the exhaust pipe. Once your child gets comfortable with the bike and wants more power/speed they suggest you remove the washer (restrictor). IMO your stock exhaust is a restriction.
I learned from experience what opening up the stock muffler could do. Also adding another silencer (less restrictive) to the stock header pipe yielded even better results.
NOTE: noise does increase when altering the silencer/muffler
Finally, you will have to be sure your jetting is correct.
You're assuming your jetting is rich. Are you sure you're not lean?
You need to find out because a lean mixture can add heat and damage the engine.
Have you conducted a plug chop to check your jetting?
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My Yerban assault vehicle: 2.2mm stroker crank, 62mm Nikasil cyl, Taida large vavle head, American made valve springs, ported intake manifold, TM 28mm carb, stock CDI, Bando coil, TK exhaust, stock CVT except for the 14g sliders, 13/40 internal gears, 16T drive, 31T axle sprocket, 22" rear tires, and ONE BIG POTATO CANNON mounted on top..........
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