Silver Arrow Bowmen has dones several repairs over the yreas and this is what i can tell you.
Start BEFORE your target gets too shot up! if your kill area is starting to flake and get shot up, the best thing you can do is buy CHEEP silicone caluking at walmart and inject into the target. Silicone is verry hard to pull arrows out of, however it binds well to the shot up foam. It will fill in many of the cracks of the target and get it to last longer.
If your kill area is cored out then the expanding spray foam is great. It comes in differnt hardnesses and expands into cracks so it work s well. Wrapping it works well. We found that this method doesn't last all that long with intense shooting.

One expiriment we did was take somethin sheet foam and wrap it like a cinamin roll and insert that into the cored out area then use the expanding foam. This seemed to work OK but was harder to get the expanding foam to fill all the voids. This also worked better when taking a deer and using it as a quartering away shots because it utilized a different angle and the different layers of foam.
IMO you are much better off buying new targets if you have let your target get past the repairability of using silicone calking. It seemed the best fix for the time/$. This is also coming from a club where targets get pounded by 200+ arrows in a weekend.

Hope that helps!