Replacing and calibrating mirror shutters is one of the hardest things to do on a film movie camera. It's easy enough to remove one, but there are a number of things that need to be checked and potentially re-set when you fit one back, including the shutter timing, the depth (or ground glass depth), the ground glass framing, and importantly the spinning flatness, which needs to be set to within about 10 microns as the shutter spins to avoid a vibrating image in the viewfinder that will make focussing very difficult and give you a headache after about 10 minutes.
Some of these settings can be retained if the whole mirror assembly is taken out complete, but to strip and re-aluminise the surface the mirror needs to be separated, so everything will need re-setting.
There is also the possibility that the corrosion that has gotten under the mirror surface has also damaged the highly polished glass substrate that creates the mirror's flat surface. I would recommend replacing the mirror, and having the procedure done by an experienced workshop, otherwise you may spend a bit of money and still end up with an unusable camera. Admittedly I am an Arri technician and not overly familiar with Aatons, but I believe the technical realities are identical.