Hughes,
I'm not sure about just welding on the pin mount, but usually it is best to modify the new peg (in this case a newer KX peg) and weld it around//over your old peg (with a bunch cut away). This way most of the old peg mount and structure (& strength) is retained, but you have the new peg boot surface. I did this for my sons PW50 with good results (sorry no pics). If you can, post a pic of your old peg so I can have a look. Thank. Cam.