Thank you for the clarification on the hex head being rounded off. I figured that’s what the problem was after looking at illustrated parts lists for your tractor and seeing how the strainer is actually also the banjo bolt.
First off I would order a new strainer and the sealing washers for your tractor with the plan to replace the current damaged one. That way you won’t have trouble in the future.
The current hex on the strainer looks pretty well non existent so the use of the proper size 6 point socket will probably never work. But you should get the proper size 6 point socket (or box end wrench) for installation and future removal.
Here are the processes that I would suggest you try to remove the existing strainer:
Pipe wrench if you have a good one.
Very good channel lock pliers that are designed for nut or round object removal.
Very good vice grip pliers (if the hex head is tall enough)
Then if it is still not loose then move onto more significant attempts that may require welding.
Weld an un-plated hex nut to the head on the top of the strainer nut using the inner diameter of the nut. Use a 6 point socket on the new nut.
Weld a flat bar to the hex head of the strainer using a round hole in the bar. Maybe the hole large enough to fit over the strainer head and weld it all around. The flat bar should be long enough to use to turn the strainer, maybe with the hole at the center of the length if there is room for that.
GOOD LUCK
