What year is the donor car?
Basically, up to the facelift in model year 1999, the immobilizer system had its own ECU (located below the instrument cluster, part number 8D0 953 234).
If you have this older system, just pull/buy get that immobilizer ECU from the same car you got everything else from (!), integrate it into the wiring and you're ready to go.
Keys, engine ECU and immobilizer ECU have to be matching. If you get an immobilizer ECU out of another donor car, you have to go to Audi - they can make them matching. That is a rather simple process any Audi dealer can do if you provide proof of ownership.

After the 1999 facelift, the immobilizer is integrated into the Instrument cluster - no more separate ECU. You'd have to re-use the A8 instrument cluster in whatever you'd want to use the engine in. Here, engine ECU and instrument cluster as well as the keys have to be matching.

It IS possible to reprogram the ECU to get rid of the need for the immobilizer. But, in this forum at least that has seen quite a few members' cars being stolen, I seriously doubt that anybody who knows these systems (I don't) would be inclined to discuss how to circumvent the immobilizer system.... that is a somewhat touchy subject around here.

Are you planning to put the ABZ in that GAZ thing for off-road-use? Cool project!