Why do you say he cannot do a KA swap? It may not be easy or cheap, but anyone can swap to any engine as long as it's the same year or newer than the vehicle and retains the OEM emissions equipment from that engine.
OK, I am sure there is a way, and I would likely cringe when I asked for the paperwork/rules from the state of California to do say a KA swap into a late 620, I would hope that following every rule in that paperwork that I could get thru emissions, but I would not hold my breath as I would likely suffocate.
Being that your in that industry and have done it in the past, you likely know some of the people in the emissions station, you likely know which station to go to and which line to get into so that if the sniffer passes it, it will also pass visual because they know what they are looking at instead of getting someone that has not a clue and it just fails because they don't know what they are looking at, or when they see something like a KA swap do they tell you to go to that line?
I already told my emissions story about my diesel truck so I know there are folks in them places that do not know what they are doing, but the point is that when something odd is brought into one of them places Like a KA swap or a Datsun diesel in my case, it is likely best if you get the right person which you likely know who the right person is if you do swaps for a living, and when I said right person, I mean a person that knows exactly what they are looking at.
I have heard a lot of stories about vehicles and emissions down there, no one talks about the ones that pass, they just go get their tags and smile every time they drive it, I am sure there are a lot of them down there, we just don't hear about them or how much it cost them either.