You would have to install standard plugs, either pin lock or ball lock. Whether or not either of these would screw into the existing holes can only be determined by experimentation. As new plugs are relatively expensive compared to used cornie kegs, I wouldn't bother.
However, if you get a few cornies AND the holes check out, you could buy plugs without worrying.
Remember one unassailable statistic, as explained by the late, great George Carlin: "Just think of how stupid the average person is, and then realize half of them are even stupider!"
"I would like to die on Mars, just not on impact." Elon Musk