I think there's little doubt that the stain on the dial is rust and the source of the rust is the movement parts. While the dial may clean, picking up a used dial won't be hugely difficult. However, I would anticipate that the dial side of the movement looks grim.
I'm in the UK and my first (and second) thought is that this would be best handled by STS. In my experience it can be less costly when multiple new parts are needed. My inclination would be to attempt to clean the dial first and, if unsuccessful, source a used replacement and then send the whole lot to STS for assessment. I seriously doubt that the cost of sorting it out would meet or exceed the value of the finished article.
Took me a bit to figure out the above statement. I think this is the route I will take, going to send it to Omega for service/ service assessment spare my watch guy some time for other jobs.
Cheers
Shawn