Mechanically it's a drop-in replacement but you will need all of the CDL's ancillaries. Boost pipes, intercooler etc etc.
The electronics is the fun part.
You can either:-
Swap in the CDL engine harness and ECU, which will need immobiliser coding and a lot of CAN component disabling (Haldex delete etc)
Or....
Retain the existing GTI harness and ECU, but you will need a remap to account for the larger injectors, higher turbo and fuel pump flow, different cam profiles, different VVT profiling etc etc.
The latter option is the least painful overall.
Are you near an established VW tuner?
It's well worth the effort though as the CDL/CDLA/BYD K04 engines are strong as war bunkers so long as they're looked after and not over tuned