So a quick Google suggests the freehub has the part number Y3B990200 and it would appear these are no longer available.
I’m a big fan of the 7800 groupset so I’d keep it going, with a new wheel or hub. I can’t think of any other parts you’ll not be able to replace for a good few years.
Whether you get a new wheel or hub and swap the bits or indeed the whole hub depends on how much use the wheel has and how smooth the bearing runs are.