My kubota came with a hole in the cab ceiling to take a radio and there is a connector there to connect power and speakers. Stereo speakers were pre-installed (well the speaker grills are there), I have never pried one off to see if the speakers are there. Adding a radio would be dead simple.
Installing a radio from scratch would not be much of a challenge but you do need to consider weather proofing issues if you don't have a cab.
Pat