It looks like the chuck has three screws on the back to remove it from the backplate, so you might be able to use those. You should keep putting oil on things like that to stop them rusting until the roof is fixed.

The 2.2kW spindle will use ER20 collets, so if you get an ER20 chuck for the mill then the collets will be interchangeable. However this does mean you can only use up to 13mm shank cutters, which may be too small, in which case get ER25 which goes up to 16mm.