There's a difference between the 922r regulations and the C&R regulations.
The Yugo 59/66 is imported as a C&R rifle, since it is not a "sporting" rifle. According to the ATF, grenade launchers, night sights, and bayonets are not sporting features, and if the 59/66 were not classified as a C&R gun, it would not be importable. Any alteration to the firearm voids the C&R status (about the only things allowed would be repair of a gas valve or a refinishing). It is my understanding that if you change anything, the bayonet, grenade launcher, and night sights need to go in order for it to be considered "sporting." Pull those, and you can swap in a scout mount or Monte Carlo stock.
The 922r law has to do with parts count- you aren't supposed to have a non-sporting imported firearm. If you want one of those, then you need to have no more than 10 imported parts- this would let you use a pistol grip stock or detachable magazine.
Now, does all this matter? Good question. I doubt you'll ever have the ATF asking to see the thing and check parts counts. However, I always recommend and advise that you follow the law in all particulars.
The 59/66 has a pretty convoluted legal status, and that's why I usually tell people that they are fun guns, but only as they are- trying to keep them in the ATF's good graces can be a mess.
As an aside, the night sights are flip-up sights. The rears are pinned right to the rear sight blade, and the front is co-located with the base of the grenade launcher sight. Mine really aren't night sights anymore- they quit glowing long ago.