I used to drive those things in the army on occasion,and due to their brakes,steering,handling,and braking,any speed above 45 MPH is just too damn fast. Yeah,you can get one to safely go faster by replacing the suspension,steering box,brakes,and the front and rear axles,but stop and reflect for a moment on everything you have to replace and what you will be left with when you finish spending all that money and all those hours on the modifications.
Rolling a boulder up a mountain would be a more sensible way to spend your time.