It is easiest to use a "3B" engine from a 1991 Audi 200 turbo quattro 20 valve. The later "AAN" engines of the US-spec S4/S6 represent a more advanced development of the 20vt powerplant, but the intake manifold design means that installation is more problematic (read: expensive). Not that it couldn't be done. For 1993, Audi moved to the "ABY" engine for the S2. This engine was a close relative to the US AAN, but I have been unable to determine whether the ABY used 3B-like crossmember plumbing, or whether it resembles the S4/6 in bringing the charged, cooled air up on the left side of the car. If anyone reading this has a scanned picture of a late series S2 engine bay, we'd love to see it. Ned Ritchie's Audi 90 project car uses an AAN engine, connected to an Audi Sport intercooler, so obviously it can be done. As usual, AIPFAP (Anything Is Possible For A Price).

Above is the engine donor car, at ABC in Leominster, Mass. Picture taken just after removal of front assemblies. The damage looks severe in this picture, but the engine was unscathed, and still running.
 

Shown below are a stock CQ engine bay, an S2 engine bay (early, 3B engine), and an RS2 engine bay, respectively.