as above, the only one you need to swap is the entire engine harness. the existing interior loom is the same, even the bit to the back of the car to the fuel pump. you will have to change the fuel pump for a GTI one as 1.4 is lower spec.
the main worry is getting the same type of engine harness as whats fitted to your car, later mk3s have the engine and lighting loom all bundled together, early models have 2 separate looms for the engine and lighting. obviously if the loom you have is different to whats in the car now, you're going to have to do some fiddling or find another loom to match.
if you have an early car with separate engine + lighting looms you might be out of luck as you may not be able to get an AGG loom that has a separate engine loom
the clock loom there are also different types. some have a 3 pin plug on for the 2 indicator wires and the mfa signal wire for the mpg, some have these on separate plugs. either way your car will probably have the wiring, just a case of matching it up. if you want the mfa you will need to add the loom between the mfa stalk to the fusebox plus change the clocks and the stalk obviously
