Potentially i might be up for repairing 'em but the usual caveats apply... I'd want to see what's involved first (pics etc), there's typically always a bit of 'suck it and see' to casting repair in that unless i've missed something we don't know that they're AZ91c, just that it's the most likely candidate and that they're very likely something from the AZ family. We don't know what temper they are etc. However based on what the owner has said I doubt he's gonna like the sound of that and i doubt he'd want me touching them anyway as i've not got any current/valid certs and the expired ones weren't specific to magnesium anyway.
As i understand it, with the AZ alloys at least, the issue with welding is as much if not more to do with the HAZ. Specifically stress corrosion cracking IIRC. Somewhere on the magnesium-elektron site there's a pdf about joining mag which covers welding and includes details of the stress relief heat treatment necessary to prevent/reduce the risk of SSC. Might be covered in the book link that Shox Dr posted too
As i understand it, with the AZ alloys at least, the issue with welding is as much if not more to do with the HAZ. Specifically stress corrosion cracking IIRC. Somewhere on the magnesium-elektron site there's a pdf about joining mag which covers welding and includes details of the stress relief heat treatment necessary to prevent/reduce the risk of SSC. Might be covered in the book link that Shox Dr posted too