That's been neglected for a long time. Personally I'd get a new axle nut, in case you need it, then spray penetrant on a few times over a few hours or more, then start attacking the one on there to see what it takes to get off. The axle shaft itself may just need some cleanup, as well as the hub, then grease them before reassembly.
The question is whether to do that now, or just wait till you need to replace the hub or ball joints or whatever and then need to take it apart anyway. In that case you could put some spray grease on it now, which will seep into the cracks better, and just clean the rust off the hub (and grease or anti-seize coat it) while it's exposed, which will at least allow getting the wheel off easier in the future and it will sit truer on the hub, and the back of the wheel itself and center bore may also need similar treatment.