Top end is probably about 105mph whatever terminal velocity is for a bike this weight , from a height of 250 feet from beechy head ha ha
If you want to make a 125 go faster ie over a hundred then you really need a two stroke and have it derestricted apprilla are famed for their 2 strokes & will go well over 120 but they are a bit more expensive to buy second hand and quite expensive to derestrict properly.Better of saving your money and if you don't already have it get a full licence and get a bigger bike.
Bandit is a good platform to start from & relatively cheap second hand