I don't know, and it would take much more to explore, but I suspect the cracks may not be the issue they seem to be. They originate in an odd place, where the braze to the tube seems OK. Cracks from mechanical stress don't originate in the middle, so I would have expected an area where there was a gap in the braze, or for the cracks to run out to the inside of the steerer. There's also an odd symmetry, and some kind of line in the same area on the opposite side.

I suspect hat these may be cracks from heat stress during brazing, or some kind of shallow defects in the crown's casting or forging. In either case they would have been there all along. Of course, rust could have made them worse or to where they extend deeper in the crown.

I'd so some exploration, and have an expert framebuilder, preferably one who's worked with cast or forged crowns take a look. I'd also probe for depth with a fine needle, and see if you can cause any movement when stressing the blades with the steerer in a vise.

If All checks out, I'd arrest the rust, then clean and seal them with either silver solder, or even epoxy resin, then paint the bottom of the crown with a clearcoat so I could keep an eye on it for any changes.