I have an
L4701. Same problem with the ROPS, as I have to lower it to store in a garage with 7' door. I definitely want it up while mowing, as I have some pretty decent slopes on the property.
I found a couple of 1/2"x3" grade 5 hitch pins at Tractor Supply. These slip right through the welded bolt head with no problems, and the factory tethered retaining pins work in the hitch pins. I keep the original factory bolts mounted in the holes used for the down position. When you lower the ROPS top, it only goes down halfway, so easier to pull back up from the cab. Very clean solution with no modification.
Though the hitch pins are smaller diameter than the factory bolt, they are grade 5. Would take a significant amount of force to shear them off.
I'm sure that Kubota made the bolts a pain to remove on purpose so that fewer folks would want to remove them.