Keep in mind that the Cobray and MAC design places the mag behind the trigger. A redesign would be needed for an AR-type lower trigger group.
Also the Cobray/MAC bolts have become rather spendy and hard to find in recent years. A 7.62x25 mm version using a Czech M-24 mag seems like the thing to do.
I believe that the square tubing used on these was an oddball size but no big deal if you can find a standard Cobray/MAC upper.