The MRI magnet is an element of a device, i.e. an imaging system. The system must be CE marked per the MDD, at least, to be marketable within the EC. Likely any manufacturer of MRI systems that would want to market in Europe already is appropriately qualified to apply the CE Mark to their systems, and no further testing would be needed to establish qualification under at least the first two listed directives, which of course are the MDD.
I'm not instantly familiar with the other listed directives, but I'd expect that similarly, if they're regarded as applicable to MRI systems, any of the major international makers of MRI systems that might want to market in Europe would already have established a qualification stance.
If on the other hand you are working on behalf of a non-European maker of MRI systems, perhaps a newer company in Asia, that doesn't have any such regulatory qualifications in place...well, I don't think working specifically to qualify a particular MRI system (or just the magnet, as you asked) as your first step would be a viable approach. I'd think your starting point would be to set up a quality system and managerial stance that conforms to ISO 13485; establish a relationship with a notified body that eventually you would work with to obtain certification for that quality system; establish a relationship with a European authorized representative; and work with your notified body and your authorized representative to obtain CE Mark ability and gain authorization for particular products.