I've been running AKs exclusively now for two years. My one remaining AR is collecting dust in the armory. For me, the AK is just about the perfect Klingon weapon offering not only projectiles, but all othe manner of hard, sharp and pointy surfaces to kill with.
That said, I have done certain things to all of my AKs. They do not change the weapon and make it run better.
1). Fuller rear sights. The Fuller rear sights have a larger notch and allow a much faster and just as refined sight picture for the times you use a sight picture. They are also the 800 meter variety and not the 1000 meter type thus are shorter and much more compact. The rounded profile is much less prone to snagging on vegetation.
2). Shepard hook spring replacement plate. One Source will eventually be making its own version of this one. It is one of the first things I replace on my working AKs.
I have seen the standard spring walk out on rifles after a great deal of shooting. The shepard hook replacement plate not only prevents this, but allows for easy and quick removal of internal parts if you choose to do that.
3). Flash suppresor. All fighting weapons need one. If your AK does not have one (and don’t cry to me about how it won’t be PC) figure out a way to add one. There are plenty of good ones around.
4). Modified selector lever. To facilitate its use, bend it out board slightly so it is easily operable with the fingers. This is an easy fix that you can do with any AK that falls into your hands.
5). G2 Trigger kit. Think of it as a drop in triger job. They are US made and easy to install. All of my Aks…even my WASRs have these installed.
That is it. You can certainly get other things for your AK, but these are the bare bones, but also essentials.