Do cataracts or the use of spectacles render the cornea unfit for donation?

No, both these conditions relate to the lens of the eye and not the cornea. In fact, people who have been successfully operated for cataracts or glaucoma or even retinal detachments can also donate their eyes after death. In fact there have been instances of those who have received a corneal transplant themselves donating their corneas after death! All that is required is that the donor have a clear, transparent , healthy cornea at the time of death.