Can this water valve be replaced as a separate component?
That's just a basic quarter-turn ball valve that's been tossed in there for shutoff purposes

The cracked part isn't part of the backflow preventer proper; it's simply a quarter-turn ball valve that's been put there to allow the preventer to be shut off as part of winterization or testing procedures.  Simply replace it, preferably with a version that has a drain cap on the side for winterization purposes, and you'll be good for a good, long time afterwards.  Note that you'll need to undo the union below the valve in the process of replacing it in order to be able to unscrew it.