As long as you have the registration from the person you are buying it from and a receipt from that person including name, address, make, model year and serial number it is all legit and KY will issue you a title.
If the registration was never signed over KY wont accept it, because the purchase receipt does not match the registration. On the registration there is a place for the original seller to sign off to whom you are buying it from. As long as he signs the registration card KY will most likely accept it but they don’t have to. Legally it should not be accepted because he is avoiding paying tax on it and other Tax offices don’t like that even if its from another state. They get accepted this way all the time but technically your clerk officer can refuse to title it for you.