Has cheap scopes or aperature sights improve your groups using your sks?
An optic or peep will usually improve groups on any rifle and improve the speed at which you acquire a target.
But to properly "scope" an SKS you must bolt a base to the receiver or it will not be stable. If you use a replacement dust cover/scope base it will move with every shot and your point of impact will move around. Some will disagree, but this has been my experience.
A peep sight is a better option but again must be anchored to the receiver. I would not want any aperture sight that must be removed every time you dissasemble the weapon for cleaning for obvious reasons, even if POI is unaffected.
The SKS is best left stock, IMO
One thing though, if you just want to improve your short range speed, one plane focus and possible better low-light performance you can mount a replacement dust cover and short tube, low power scope. Should be acceptable at around 50 yards or so. Just don't expect pin point accuracy at longer ranges.