I have found that all bx's run hot during the summer, mine needs a daily rad washout when I'm mowing, sometime several times a day when it's really warm or dusty.
Putting the rad behind the motor and above the mower was not Kubota's most stellar moment, besides which the screen in front of the rad might well stop an errant gum wrapper or the occasional stray cat but the rest just goes straight through and gets stuck in the rad fins.
Changing the thermostat might help but I think that it's mostly just dirt and it don't take much as the radiator is just too small.