I just bought a brand new Hungarian AK.  I don't know anything about AKs.  Just wondering if there are longer barrels available or a long barreled upper?  I am also curious about what would be the best side mounted scope system.  Any help is greatly appreciated.

No not really. AK's don't have uppers, they're pretty much a one-peice unit. Changing a barrel isn't much easier than building a whole new rifle.

The RPK is a light support weapon that is based on the AK and has a longer, heavier barrel.

As for the scope, a PSOP with the 400 meter retical should work. They're supposedly calibrated so that if you zero the top chevron at 100 meters, the 2nd and 3rd aiming chevrons are accurate 200 and 300 meter trajectories.

So could I build an RPK with my AK receiver or buy the whole rifle?  Does the RPK have a longer barrel?  I'll check out the PSOP, I was just looking for something to mount a Red Dot on.   Thanks for the help.

Yes, the RPK has a longer, heavier barrel. The AK receiver will not work to build an RPK on, the RPK uses a thicker receiver with bulged front trunnions and a different rivet pattern. Before you proceed, you should understand that AKs do not just bolt together like an AR. They require 'manufacturing' with special tools including a press of some sort, and riveting tooling.

Right now, that I know of, nobody is producing a complete RPK that you can order through a gun shop. There are several places that can build you one though, however you'll have to supply them with a parts kit and a receiver. And probably compliance parts.

You can buy mounts from Kalinka Optics to put a red dot on, or you could just buy a Kobra, which is a Russian made red dot optic designed for AKs that fits on the side rail.