3 options, in order of simplicity and performance increase

1. Get a reducer pipe and fit a GTI system to the 1.6 downpipe
2. Get a 1.8 carb downpipe & GTI system. The 1.8 carb downpipe bolts to the 1.6 manifold and ends up the same bore as the GTI system

3. Fit a complete GTI system including manifold & downpipe