Per CNN it is five acres, compared to 28 acres for the original park. You should be able to squeeze in four attractions the size of Taniwha Tubes, although I think three would be more likely. Part of it will depend on things like adding in additional lockers, restrooms, food service, etc.
It would be crazy expensive to retrofit at this point, but if the entrance could be moved, and some sort of access tunnel built, the entrance area could be used as a last-ditch expansion plot.