I am getting ready to place an order for a Kreb’s stretch Kinkov and am debating the caliber. I prefer the ballistics of the 5.45x39, but am hesitant to shoot a ton of steel cased, lacquered ammo through a $1200 rifle. So I am leaning toward the having it built in .223.

The thing is, I have a ton of East German Bakelite mags in 5.45.

Do I need to modify the Bakelite mags for them to feed .223?

If so, why and how?

The Krink will likely have a chrome chamber/bbl and thus no problems at all with commie steel case ammo. Your short gun will also likely be more fun in 5.45.
For 5.56 ammo in 5.45 mags, some followers are needed from Robinson Armament (VEPR importers) and some dremeling of feedlips. Or about $20 on specific 5.56 AK mags.