Stock carb is a phva14 which = 14mm
A 17.5 carb will fit on stock intake but the stock intake is only 15 mm at its largest and cylinder intake hole can be only 14 mm
So you will need a aftermarket intake and dremel the cylinder intake hole to 17.5 mm.
You should be able to get low to mid 40s mph out of the stock 14mm carb with the right filter and pipe.
What is your set up and what is your speed?