While you're free to spend your money however you like I would like to point out that the Swiss Arms magazine design is actually stronger and more reliable than a STANAG pattern magazine. The money you would spend on the replacement lower would buy you a few more SA mags and still leave you money for more ammo.
I understand the thought pattern of wanting one magazine for all your rifles but in this case and especially with some cheaper aftermarket magazines available it just doesn't make a lot of sense to spend that much money just to make your rifle use a crappier and less reliable magazine.
When I had my Classic Green I had 9 SA pattern magazines for it (mostly Speer) and while I do find the mag release lever and rock in motion a little more cumbersome than my AR's it's nothing you can't get used to and with some practice just as fast with.
Sorry, I don't know who carries the lowers.