You can change the sprocket, if you go smaller you get better top speed and less acceleration if you go bigger you get better acceleration and less top speed. Bit of a trade off but easy to do, as for "how fast will my bike go" it's a 125 so your about at the limit, you could lose weight take the junk out of your pockets or strip everything you don't need off of it to get a few more MPH but really how fast do you want to go on a 125? We have had loads of folks on here who want to get really high MPH out of their bikes and we always tell them the same thing, pass your test and get a bigger bike, the reason is simple if you thrash the shit out of your bike till it's screaming in agony it will blow up, not straight away and it might even last a few weeks but it will destroy itself eventually, no matter how much you spend or how good you are at maintenance it has a limit and if you pass it your gonna be stuck at the side of the road waiting for a big yellow truck.
If your in the 16 to 20 year old age range then your going to ignore me and say I don't know what I'm talking about and you know best, ok fine by me but most of us on here have been where you are now and from personal experience I can say treat your bike with respect, accept it for what it is and deal with the bike as a whole, don't try to "improve" it, cos if Yamaha could make a cheap 125 that would do 100+ MPH they would.