An old-time radiator shop may be able to fix it. It looks far gone though.
The other alternatives are to find a similar size tank from some other application or to just have a new tank made. But a new tank will be expensive if you pay someone to make it.
if its installed someplace not terribly visible on the tractor with all covers and hood in place, you may also be able to put a smaller good tank inside the rusted out shell.