They are 4" speakers. I agree about the terrible quality. One problem regardless of the quality of speaker is the fact that the headliner covers them (unless some of yours are different than my father-in-law's). I just got done adding an amplifier and two DDAudio VO-MN6.5s and two VO-B3 compression tweeters. That Axxera radio that was in there is still junk but this set up can be heard easily above the engine noise.
Any 12v radio will work in these. Don't let your Kubota dealer tell you theirs are something special. Jensen and Axxera are the two brands I've seen in these and neither are very good quality units, especially for the price these dealers want for them.
Here's a link to some of what I did to this one:
https://photos.app.goo.gl/4nWsfxytQRph1EqR7
I used the existing holes for the cable ties to put riv nuts in the front spots. I then drilled my own hole in the back to put a riv nut (not pictured). I trimmed then bent then mounted a 1/4" piece of ABS plastic to those holes. After a test fit I cut holes in the headliner and then into the plastic. I tapped the plastic and mounted the 6.5s with machine screws. The tweeter has its own mounting hardware, almost like a giant nut. These speakers are powered off of a DD Audio d4.75.
I also did a remote start on it.