I noticed our backflow preventer was leaking, so I pulled out the assembly to see what the issue is and it looks like a decent-sized crack in the plastic above the relief valve. Is this something I can seal and have it hold, or will I need to buy a whole new assembly?
I would not expect success attempting to glue or seal that. Apart from the general difficulty of "plastic of unknown type" repair, it's usually the case that when a plastic item cracks one place, other places are likely to follow, even if you could get this crack sealed successfully. They do not age gracefully.