Get out your checkbook.
The Yugo 59/66 SKS is imported as a curio/relic firearm. It is only a C&R firearm so long as it is not altered from its original state (replacing the magazine with a different type or mounting optics are such alterations). Once the C&R status is voided from alterations, the firearm is an imported non sporting firearm, which is prohibited from ownership as per
section 922r. This means you are in violation of federal law unless you replace several parts of the firearm- you can have only 10 or less imported parts on a non-sporting firearm. You will need to replace seven parts on that rifle with US-made parts in order for it to be legal.
The Yugo has a pretty funky legal status, more convoluted than that of even other SKS carbines. Many folks don't think it a big deal or scoff, but the law is the law, and it is foolish to willfully flout the law, no matter how obscure or difficult to check. I recommend following the law in every particular.
You will likely need to replace the gas piston, operating rod, magazine (three parts there- body, floor plate, follower), stock, and hand guard. All those parts are fairly easily swapped and will put you into compliance.