I have a converted Saiga and have had both a Russian 6x PSO scope, and a Russian Kobra red dot sight on it, both of them using the normal AK optics rail on the side of the receiver.
My thoughts? Both of them were very heavy, and the Kobra wasn't nearly as bright as a BSA I saw at the range that day. Add 20 rounds of 7.62x39 to that, and it was less appealing than I imagined it would be.
If you are looking for a practical shooter and not an authentic looking Russian package, I would go with a quality commercial red dot scope attached by means of a "low profile" side mount, most likely with a Weaver rail on top. Stay away from anything that mounts on the receiver cover.
Also be sure the mount you buy is for an AK, not an SVD/Dragunov. I almost bought the wrong one!
-jacob