I just encountered this scenario recently, To rebuild the physics bodies and their constraints, you can reference ue4 engine code initializing these physics elements. You can search and checkout this function : USkeletalMeshComponent::InstantiatePhysicsAsset_Internal
and ue4 first evaluate bone transform using skeletal animation, and conditionally blend it with their respective physics body transform, you can check out this funcion:
USkeletalMeshComponent::PerformBlendPhysicsBones