Prop flange rust?

Mine started to develop a little surface rust, not as bad as yours, I just wiped it down with some Tectyl 506 underseal - problem solved.

Oil will also do the job but is nowhere near as long lasting as a wax based protectant.

Every 100 hours, you should treat the propeller flange with a corrosion protection spray. ACF-50 or LPS 3 are good products to use. Some environments can cause the prop flange to rust more quickly than others, depending on how the aircraft is stored (inside, outside, etc.).

That said, Rotax is aware of the complaints, and hopefully, they will come up with a solution. It’s definitely frustrating to spend a small fortune on an engine, only for it to look like this after a short time.

At this stage, is there anything I should do about it?

Should I spray a coat of ACF50 on it? 

Is there any easy way to clean the rust off before I do? Would spraying it with Corrosion X first help remove some of the rust, then coat it with ACF50? Or just put a coat of ACF50 on it?

I was planning to have it bead blasted when it’s off at the next gearbox inspection, but that’s still probably two years away. 

Is there any structural concern with this rust? Or purely cosmetic?

If Rotax is aware of the complaints, doesn’t seem hard to fix going forward. They could just epoxy primer the new prop flanges they make? 

"Is there any easy way to clean the rust off before I do? Would spraying it with Corrosion X first help remove some of the rust, then coat it with ACF50? Or just put a coat of ACF50 on it?

I was planning to have it bead blasted when it’s off at the next gearbox inspection, but that’s still probably two years away

Is there any structural concern with this rust? Or purely cosmetic?"

Yes! It looks to be cosmetic, at this stage. Using a wire brush, remove what rust you can, (be careful not to damage the shaft seal ) and then treat with an inhibitor like ACF50 or Tectyl 506 (sprayable wax)