No way to know unless you are inbedded within the Toyota production pipeline.
They could be or availability is now down to remaining shelf stock. Do remember that the very newest vehicles that use this part are 36 years old.
Remember his answer will reflect what is currently in the US and in transit to the US. This will not reflect global stock or any corporate decision to discontinue production.
Safety recalls. However they are not obligated to supply parts forever. Once parts are gone the manufacturer will offer to by the vehicle back. The owner can choose to keep it.
I do not know the parameters. My guess is it would need to be a functional licensed vehicle. If it doesn't otherwise function a new tank makes no difference.
True a tank could be modified but the only way to get a new one is if it is supplied to an FJ60 with an uncompleted recall. Modifying it doesn't work if you can't get one.
At this point The only obligation is to buy the vehicle back. You don't get to pick the number. You accept it or keep the truck. A buy-back will be crushed.