It'd be as straightforward as an engine swap can get. You'll need vagcom to sync the gauge cluster to the 1.8T ECU also.

 

Full donor car probably strongly recommended?
B5 A4 is a 1996 2.8 manual trans chassis
what 'donor' car (1.8T) manual should be selected for 'good match'?
I have 1998 AEB; limited ECU capabilities, no MAP, nDBW (but strong block).
Get newer donor for wider selection of tune (with MAP, DBW, etc.)?