First of all, I do know that need can be used both as a modal and as a normal verb. (I also do know that need as a modal verb is now commonly used only in negative forms, as the opposite to must.)
Could is the past form of the modal verb can.
What is the past form of the modal verb need? Is it needed -- like the past form of the non-modal need?
2. We use the modal verb need in questions and negatives.
I don't know if it's possible to use it in positive declarative sentences. Someone else might know. I suspect, if it's used like that, it's rare and archaic.