The 28's stock is 15/44 so it is already pretty low gearing.
I'm in top gear and at peak power just over half way down my regular track's back straight, By the time I get to the braking point I'm well into the red and waiting for the thing to blow up or run lean.

Next question, are the 21 and 28 sprockets the same?
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If you want a proper 2 stroke get an RGV.