The K2 website has a binding technical manual, link was found at the bottom of the K2 home page (see link below). The manual provides NO part numbers and NO repair procedures, but the repair is fairly easy.
The back straps on the K2 Cinch are called "grab loops" as shown in photo below.
If both “grab loops” are broken, take a few photos first so you can recall the routing for installation of the new “grab loops”.
These "grab loops" are held in by two silver coloured roll pins which can easily be tapped out with a small nail & hammer. If the pin doesn't come out easy, tap from the other side of the pin. The pin has a head on one side.
You may need to use pliers to remove the broken piece of strap from the top lever.
Take the broken strap pieces to your K2 retailer ( snowboard shop) and ask them to order a new strap. K2 info suggests you can’t order parts online direct from K2, so you have to order through a K2 retailer.
Use your other binding with the good strap to copy the routing of the new strap. You might want to order two new grab loop straps and change them both while you are at it. ??