I have a number of original GI mags that I got with guns Ive bought. I still use a few of them and whenever I have a problem with the gun feeding or a malfunction, those are usually the mags in the guns when it happens.
I bought a bunch of the Korean mags when I got my first carbine, and they have worked great. They are also about half the price or less than a used GI mag too. I think I was paying around $12 for a 15 round mag. The couple of 30 rounders I have, were a bit more, but still very reasonable.
Ive read a couple of articles about the carbines and they stated that the mags were always the weak point and complaint with the carbines, even when new back in the 40's, and most mags were ditched for new mags each time they were resupplied with ammo.