The Hardy stove is an open pressure system. So you could weld it without worrying about it blowing up.
As far as what to weld it with. It would depend on where it cracked. They have a 16 gauge inside layer and a 26 gauge outer layer. So that is some real thin stuff to weld. The thin stuff is just about going to have to be welded with a TIG setup. The thicker inner layer could be welded with MIG easily.
I used to have a wood water boiler. I looked at the Hardy stoves. I thought they where too lightly built. I know they claim that the Stainless Steel makes they last and the thiner firebox is more heat efficient. I just did not think the stove would hold up.