Most below is assuming that a Passat (2.0l, 9A), Corrado (2.0l, 9A or 1.8l, KR), Golf 16v (1.8l, KR) is being used, so throttle body on inlet manifold enters on back left of engine bay, rather than back right on the mk1 GTI (and 16V Scirocco).
The Audi 2.0L 6A engine can also be used, but in the Audi it is mounted longitudinally, and therefore requires changing of all bolt on parts for mounting transversely. See the "6A 16V into Mk1 or Mk2 Conversion Guide.pdf", taken from the FAQ section on: Club GTI
Attachment
6A 16V into Mk1 or Mk2 Conversion Guide (From CGTI).pdf
There are other main option is the ABF 2.0L 16V engine, mainly found in Mk3 16V GTIs, but also in late 90's Passats and likely in Longtidinal form in Audis with an alternate engine code. I won't cover this variation here specifically as they have taller blocks and I don't know detailed differences.
