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It would put more stress on the tranny, but since the Z250 is a low power machine to start with I highly doubt it would damage it. Its the piston, crank and bearings that I would worry about.
Don't put a NOS kit on a Z-250. Save you $$$ for more logical "all the time power", instead of a limited burst you can only use at WOT. I don't thing the motor would like repeated shots, even if if was a small shot.