I'm sure someone can and will correct me, but my understanding is that they at least have parts for 3 trains for Viper, originally had 3 trains for Tantrum, and 2 for Predator, but they've just used all of them for parts. As someone who has never done maintenance on a coaster train I don't understand why they can't get 2 trains running on Viper and Tantrum (last time I was at the park Tantrum was running 2).
I'm sure the other trains have been parted out pretty extensively though and I'm sure it would take a lot of work to get the other trains running, but we can wish.