I have no idea of the proper bolt size.
I do know that for some reason Kubota LOVES to use extra fine threaded bolts, most of the M12 size. This bolt size has three thread pitches.
Coarse Fine Extra fine
[FONT=Arial, Helvetica]M12-1.75[/FONT][FONT=Arial, Helvetica]M12-1.5[/FONT][FONT=Arial, Helvetica]M12-1.25[/FONT]
There seems to be a break point on that tractor (I looked only at the 2WD version) since the parts list shows two part numbers:
01133-51435 or 01133-51440 BOLT,SEMS Qty 4
(SEMS bolts just have a washer already attached)
Your dealer may have them in stock if your have one locally.