The Honda XR125 will have a top speed of around 55-60mph which in some instances can be frustrating for the rider as you lack the ability to over take at the top end of things.
The Honda XR 125 is a 4 stroke and cannot be derestricted because it runs at full capacity as is. The only way you are going to get more speed from your machine is to drop an engine in from a bigger bike.
I guess that is an option to someone who is not licensed to ride anything more than a 125cc bike, but if you are then just buy the bigger bike and save the hassle.
My Honda XR250, (the one I am rebuilding as a project bike here) will happily sit at 70-75mph so isn’t a great deal faster but these bike aren’t built for top end speed just like the 125 isn’t. They are great on acceleration.
One thing you can do to transfer more of the power onto the top end is play with the gearing. This is done by swapping out the front and rear sprockets for different sizing to gain performance.
Buy decreasing the size of the rear sprocket you could gain a little on the top end perhaps, but bear in mind you will loose power from the opposite end, in this case lower end which means acceleration. I doubt it is worth the effort.