Can I repair this crack in my sprinkler backflow preventer?
It's a fairly standard quarter turn ball valve, for permanent repair, take the union loose, remove the valve and replace with like.

No patch you can apply will properly seal against expansion propagating through the patch material or breaking the patch material off eventually. Any moisture underneath the patch allowed to freeze will spall it off of the casting. Water pressure under the material tends to work it off akin to how the pressure in a blister separates the skin layers. 

About the only thing I would recommend for patching is MarineTex which is a super duty industrial Marine Epoxy made for use on water containing castings in damp environments. You will have to sand the area to remove casting skin, extending the area beside and well beyond the crack, clean the surface thoroughly with MEK or Acetone and then mix up and apply the MarineTex per directions.