New Zurn Wilkins 1" Pressure Vacuum Breaker to used to prevent backflow in irrigation systems) - I "borrowed" the seat assembly from the new unit as that was the only thing that need replacing on my existing 420 unit. Didn't have to deal with any plumbing issues. The only part needed is Zurn 423-180 which is part of repair kit RK1-420 (available on Amazon...see photos). See photo of the unit I replaced...looks like a basket (I kept it for future reference and will send it with the purchase).
You can purchase this for parts for your existing unit or buy the missing part online and put on the whole NEW assembly.