If you are changing to the single PE temp sensor , you only need the top access hole for your sensor . The standard radiator has two locations for the two original sensors. The original sensors will be very tired as will the mounting studs in the radiator. if you can get the nuts off the embedded studs in one piece then pulling out the old sensors is easy. Try not to break the studs. If you do you will have to drill and tap into the metal base boss surround and put in another stud.
You could have a special cover plate made and drilled for the second opening or get two of Jons treaded adaptors , one for each sensor opening. and put a flush SS or HD plastic plug in the lower one. The size and thread is the same as a standard marine deck drain plug, so easy to get.