Looking at the basics, they compare rather closely to a copper dragon for damage output, so we could model its physical ability scores off the copper.
HD puts it about on par with the copper as well. Size is also comparable to copper. Its AC is one better.
Int is comparable to a green, so we might model its physical ability scores off the green.
Land speed is comparable to a blue, bronze, copper, green, red, and silver. All are 40 ft. Burrow speeds for true dragons range from 20 ft. (for the blue) to 45 ft. (for the li lung). Since burrowing is a primary tactic of the stone dragon, I'd recommend going toward the high end.