You can modify a pre-ban magazine to work in another rifle, but it must still be able to function in the weapon it was intened for. For instance, Armalite is forced to ensure the M14 mags they modify for the AR10 still function in an M14. To make things easier and cheaper, they just replace the magazine body with one of their own design that functions perfectly in both rifles.
You can replace or modify any part of a pre-ban magzine including the magazine body. Most places offering replacement mag bodies swap the new one for your old one, just to keep things honest. I think it would be easier and 10 times cheaper to modify an inch L1A1 mag to work in an Saiga.