Anton, your engine has a solid lifter head and unfortunately isnt a heron type head. some type of larger exhaust, a modified / better breathing camshaft, upgrade the throttle body as preeny has said along with some de restricting of the inlet will give you some more power

Next would be increase the compression ratyio or as suggested go to a 1.8L engine ( same as whats in the green scirocco) Have plenty of books etc if you want to do some studying up !!