The barrel is chrome lined so a little surface rust is no big thing.
A good cleaning and lubrication and it should work just fine.
Buy a new upper receiver. Not even an ATF part. Clean up the lower and you’re good to go.
I have a Russian SKS. I picked up some very nice Czech 7.62x39 [brass case] imported by Steyr. I took it out and shot it, cleaned it completely, and put it in the rifle rack. About three weeks later I went to the rifle rack and tried to open the action. The bolt was stuck. I put the butt on the floor and gave the bolt handle a smack with my heel and the bolt opened and locked back. When I looked in, there was all kinds of green corrosion growing in it.
I had discovered that the Czech ammo was military spec and used corrosive primers. Fortunately the SKS and AK rifles have their critical points chromed plated for this exact reason. I went back to the old ritual of cleaning out corrosive primer residue: clean and lightly oil the rifle the day you shoot it and repeat for three days thereafter. The SKS was unharmed. I traded off the Steyr ammo shortly after. If I'd fired this ammo in a Ruger Mini-30 Ranch Rifle, I'd have seriously damaged it from that corrosive ammunition!
Note to self: All foreign ammunition before 1989 should be considered corrosive until proved different.