Theoretically, you could make really cheap (but ugly) uppers. I don't know if the AR18 bolt is dimensionally the same as the the AR15, but even if it isn't the system is cheap.
1. Upper receiver. Sheet metal stamping, folded and heat treated, just like an AK. Might have to weld in rails for the bolt carrier.
2. Front and rear trunnions. They're riveted on the AK, but for US aesthetics, they'd probably be welded. You need the trunnions to have the front and rear pivot/lock points. The front trunnion gets the barrel. These would be relatively simple machined items.
3. Its been awhile since I looked at an AR180 bolt carrier and bolt, but they follow a somewhat familiar pattern. Big, square bolt carrier with a gas piston sticking off the front. Bolt is a multi-lugged, rotating bolt, cam actuated, just like the AR. No gas rings. Dual recoil springs contained in the upper receiver.
If he is using surplus AR18/180 parts, he probably has to fabricate an new sheet metal upper receiver that will match up to an AR lower. I would suspect he's using the same front trunnion, barrel and gas system, though if you were doing one from scratch, it would be economical to use a front trunnion that would allow the use of a stock AR barrel.