“remove the internal hard drive and put it into a new iPad “ can’t be done. there is no internal hard drive etc. All the information, files etc., are stored on IC’s (Chips) that are soldered to the logic board. There is no way to recover any of this. Consider this a total loss, sorry.
@kmab you know just for completion sake, post some good pictures of the iPads logic board with your QUESTION. Let’s see what we are dealing with here. There may not be any corrosion etc. on it and maybe it stayed well preserved while buried in the ground. Now, don’t go ahead and just rip it apart, use the same care as you would if it was new. Use these guides iPad Mini GSM Repair to work on it.
Adding images to an existing question