Ask someone local who knows to come look at it.
Most OWB's are open units meaning that you can't pressure test them very easily.
The lines are pex so freezing isn't going to hurt them. They will just expand when frozen and go back to shape when they thaw. It is possible to pressure test them in most cases. If they installed shut offs it is even easier, again someone local is your best bet.
The water jacket can freeze in several different ways. It could freeze from the top down which could in theory pop a weld on the bottom but that is highly doubtful unless it was a weak weld. OWB's are usually made of decent sized steel and will take a fair amount to break a weld ( age of unit and water properties etc make a difference) It could also freeze from the outside in again doubtful that it would pop welds. Most have an open top on them to add water and a place for excess to go if they boil over. Also very seldom are OWB's filled to the brim so doubtful if it would pop welds. Think of it as being a steel barrel you leave full of water in the winter, usually the ice just pushes up and if it's a thin barrel it might distort some but not likely much. Now if the barrel is sealed and there is nowhere for the ice to expand to then it will break wherever it is the weakest.
The pump is kind of a sealed unit so yes it would freeze, expand and crack the housing.
I bought a used OWB that sat outside for years with water in it, no leaks and no issues. I replaced pumps, thermostat, the fan and fired that baby up.
Spent the few bucks on a pump or jury rig something up with a sump pump and get the water moving.