I love the gun, but I'm having problems with the original magazines - 10 years old - the seam on the back of two of the magazine has opened a bit and it's affecting how the follower moves up and down within the magazine - the fasteners are good and tight so the magazine isn't "opening" because of loose screws - I'm wondering if there are any steel magazines available for these guns, or perhaps just another more robust mag springs would get it working better?

There are no metal mags for it. Your options are factory, promag,and the 110 round drum.
The promag are ok but they will eventually split also.
The only fix you can do is super glue them together.
I highly recommend beforehand you sand the follower and inside the mag body before you glue it shut.

Also note it will eventually crack open again at some point.