Axle is around 40mm, nothing about this frame is standard, has been an issue from the beginning.Flattop has great advice - I would spray penetrating oil on it and let it soak a little bit - may be some corrosion due to dissimilar metals. Then ‘persuade’ it loose.
If that doesn’t work - replace the entire unit - what size is the axle ? You may have better luck finding a used hub and rotor. Plus, finding a replacement rotor may be a little difficult for that style brake hub. Do you happen to know the brand ?