You can try to use Apoxy clay. It is 2 component clay, You mix part A with part B and from this moment You have 2 hours until it become hard. You don't even need any glue then, it sticks to PVC quite well :)
Another idea would be sculpt missing part in Super Sculpey, then bake this part in oven (but not the whole animal) and then glue part to animal. In such situation You would need glue but I don't know which type.
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Anna
Horse and Bird studio - Horse sculptures
My model horse collection