Original SKS rear receiver covers (not the aftermarket railed models) seem to fetch a pretty penny used, a lot of people seem to try the scope mount thing only to realize it makes the gun ackward to shoot (the see-through versions are the worst) when you factor in eye relief but have often discarded/lost the original... making them particularly sought after in the aftermarket.
http://www.amazon.com/UTG-Tactical-Tri- ... 18&sr=1-13I know a gent with one of these and he liked it a lot, gives the rifle more of a "scout rifle" look and his cheap Barskas red-dot held zero without issue over hundreds of rounds so far. Eye relief is not an issue with a pistol scope or most red-dot/holo sights.
You can also purchase a rear aperture/peep sight from Tech Sights that will greatly improve the sight picture (and radius) which will improve accuracy but add no weight or a requirement for batteries to operate the rifle (no co-witness option I'm aware of if your batteries die).
http://www.tech-sights.com/sks.htmI personally like the $60 Tech Sights upgrade over most everything else out there, totally reliable and a major upgrade over stock sights without the issues of fit/finish/reliability that the lower end red-dot/cheap pistol scope + SKS mounts provide. For a range toy not a big deal, if it's a weapon you want to keep for HD/defensive purposes I'd absolutely avoid any of the other optics upgrades and just add the rear peep sight.