'91 Jetta Performance Ideas?

I have a 1991 Jetta with a 1.8L I4 motor, it has an auto trans. just wondering about what mods i can do to the motor for cheap, i heard about gutting the airbox, can anyone give me directions on this? and i also heard about changing the injectors to vr6 injectors? is there anything else cheap i can do? exhaust is going to be done soon, strait pipe and high performance muffler and at some point a header.