Hey mate, welcome.
The short answer is to swap in an SR, you need basically an entire SR powered car as a donor. Engine mounts, gearbox, axles, loom, radiator and even little things like the power steering hoses will need to come from an N14 GTI. You'd be much, much better off to buy one of those as a starting point given how little they're worth now - and then you get things like a nicer interior and rear disc brakes with it too. If you're dead set on keeping your car, and you have friends who can weld, it would be fairly easy to turbocharge the GA16 - although you'll struggle to find a heavy duty clutch for it.
If you'd like to join as a full member, have a read through the build threads section - there's a great range of builds on there so you can see what other people have done.
