Aliyanna, I'm not an expert on natto, but I have a feeling that rice (at least rice alone) might not be the best substrate. I kinda sense that it might go awry because the nutrient profile is so different than soy. But I imagine other legumes would work...but could be wrong about all of that.
The natto starter from Gem Cultures goes a long way. If you like natto, I'd be inclined to just experiment and see what happens.