I think I would take a wire wheel on my grinder and rough up the area and reduce the thickness a bit but not all the way thru and then JBWELD it. Be sure to clean the area well with some Dawn dishwasher soap and water first then, dry it and just before applying the JBWELD, clean it well with some acetone to soften the plastic and remove any greasy residue.
Remember that JBWELD will creep on you so it needs to be flat where the repair is. You may need to apply a second layer to get the buildup needed if it creeps on you too much.
I have used it on a plastic diesel tank and it is still holding after 5 years but there is very little pressure on it as opposed to the 10-15 PSI or so on your tractor's radiator.
It is worth a try but if it doesn't work, a radiator shop might be able to make you a solid brass one for less than a Kubota plastic replacement. I did this on my John Deere garden tractor. It wasn't cheap, IIRC about $200, but it got rid of the plastic end pieces.
NOTE: This pin hole leak might be just part of a linear crack that may not be visible to the naked eye. I cant imagine plastic having a pinhole leak like a metal one might get from corrosion.