Regardless of what purpose that object on top serves, I assume that it would still have been removable and that the OP would s want to remove and replace it if possible.
I went to
https://apps.kubotausa.com/illustrated-parts/ and found this descriptive term for item 200 = CAP
If I was in the predicament that
@Rob18530 is in here is how I would attempt to resolve this issue
Spray the heck out of the thing with your favorite good penetrating oil (PB Blaster, Kroil, Mouse Milk, …) and walk away.
Then order the part number 70060-00840 from the dealer. ( Messicks says it is a vent plug.)
Then every day waiting on the part spray that joint again and again
When the part arrives see how it is made and from what material.
Since I don’t weld very well nor have a welder, I would try to get a qualified welder to attach a large nut or steel bar to the top of the plug if the plug is weldable material.
Then try to get it unscrewed. I prefer to use the nut over the bar myself but have seen both work.
Just be careful not to weld the plug to the case.
If that is successful then replace the plug and use anti seize on the threads.
If not successful I would then try to start drilling the existing plug out starting with a small hole and working up to the diameter of the female threaded hole. Then see what happens and get the remaining material out or tap the hole with the required threads to match the new plug.
Good Luck