First you need a 7.62x39 upper. That's the easy part, and there are a few different choices out there.
Next you will need magazines. When Colt first started selling 7.62x39 ARs, they just provided them with a 20rd straight GI 5.56 mag, and told you not too load too many into it. If you load too many, the steeper taper of the M43 case causes the top rounds to start to nose under and it won't feed reliably. This solution works fine if you don't mind having 12-13rd magazine capacity.
You may also need to replace your hammer spring with something heavier to fire the notoriously-hard primers of milsurp 7.62x39 ammo. If you are buying American-made, commercial brass-cased 7.62x39, or reloading your own with standard primers, that step may not be necessary.
There are probably some US-made 7.62x39 magazines made these days that will work better than the old Colt solution.
If you're willing to use a different lower, then the "AR-47" type lowers have a different magazine well which accepts standard combloc 7.62x39 magazines, and that nicely addresses any magazine and feeding issues, at least.