I dont really know, if the agility 125 is diffrent from the agility 50, when it comes to how the carburator is build, but with the 50-version, the screw is made of brass and placed on the left side of the carburator. Be carefull not to tighten it, as these screws have very fine threads and will be harmed if you are too harsh.
On the Agility 50, there are a hole on the left side of the scooter (when all shields and seat are on) right over the passenger foot support, and through this, you can access the mixture screw, if you have a long necked flat screwdriver.
But remember, that the mixture screw are only dominating when you are idling or at
very little throttle. When giving more throttle, things like the shape and height of the needle are meaning more, and when at full throttle, its only the main jet, which are active. (Well not true, but the other parts are made for passing very little fuel at very low throttle, so the main jet is dominating)
On the picture below: the axis going left to right is how much throttle is given and the axis going up is the flow of fuel, compared to the flow of air.
Dark blue = mixture screw, light blue = idle jet, yellow = needle diameter, Orange = needle height, red = conic shape of the needle and white = main jet.
I don't know, if the design of your carburator is diffrent, but on the agility 50, the needle cant be adjusted, and it is only possible to fine tune at idle and Wide Open Throttle.
