922r can be confusing, I'll try to help.
Your S-12 has 14 "counted" parts. Adding the pistol grip makes the gun "non-sporting" and therefore subject to 922r, as you have surmised.
The Tapco stock replaces one foreign part so you then have 13. You still need 3 more, since you can have no more than 10 forign parts on a non-sporting weapon. Some guys count on US made magazines for compliance (since they are comprised of 3 parts) but it's iffy. Staying away from the FCG this really only leaves you with: 1) Handguard, 2)Gas Piston, and 3) Muzzle Attachment.
Does that make sense to y