You can only push the limits of the capabilities of any buggy so far. They will only go as fast as the design, engine performance and drive gearing will allow. Some mods will help achieve the maximum performance but what that maximum is will differ from machine to machine. The 250cc engine is really only able to produce "X" amount of power and most buggys will only go 45-50mph at the most.
With the mods that you describe is should do between 45-50mph. You can add heavier rollers or sliders than the stock ones and maybe get more mph but your low end torque and acceleration will suffer.
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