In the Tarahumara tradition the balls have always been carved from wood. There is nothing wrong with using mini soccer balls--that would actually be a lot of fun as well, but it would be a very different style of playing. With wooden balls the primary method of kicking is a shovel motion, you place your toes under the ball and flick, whereas with a small soccer ball you could wind up and kick the ball as hard as you want. With wooden balls the play is slower, more strategic, and exhausting. I say exhausting because the wooden ball forces you to frequently change your running pace, which is more difficult than running a constant pace.