the picture looks like the wheel and hub are one piece, I would pull one of them and see if a 4 or 5 bolt hub could go on the spindle and if it can put new hubs on and then get the rims you want and tires and put them on,
if the spindle is not interchangeable, most like one would have to cut off the old spindle and weld on new ones,
pull off the wheel or a better picture of the wheel may help, and measure the spindle and length to any machining and repost (one will just have to search out to see if there is a hub that is inter changeable), possible the manufacture has an option, but by posting a picture and the measurements some one may just know,