I would doubt that you will be able to modify it without running into a warrenty/liablility issue. Meaning if you do anything to it, and then roll the tractor and it fails, your widow would have no recourse.
If you want a folding ROPS you might just want to see if you can order one from Kubota and replace yours?
If you live on flat land and do not use ground engaging equipment (like a plow) that will easily flip a tractor even on flat ground, and you don't have a FEL, that can also tip a tractor, and you are hell bent on a fold down ROPS, and you can weld and have proper tools and design experience, and you are willing to take the risk that your design/build will hold up in the worst situation, then you may choose to go for it.
There are a lot of us who have modified our tractors, but we accept the risk of the modification. There are a lot of us who have antique, or just old tractors and/or grey market tractors, they don't even have some basic safety equipment. Any tool comes with a certain level of risk, any modification can reduce or increase the risk level. The owner makes the ultimate choice of what risk to accept.
Be aware that when you sell the tractor 2 or 20 years down the road you should disclose the modification and have the buyer sign a waiver on the bill of sale saying he accepts the liability and holds you harmless.
