From what I've seen the 1.3 responds well to a header and exhaust and a Weber carb. A full Janspeed system with a 4-2-1 header gave about 63bhp at the wheels (same as Suzuki claim at flywheel) and make made peak torque about 1000rpm lower. Different makes of headers will respond differently. You should talk to the manufacturer and ask them what the exhaust is designed to do. I doubt if the K&N will make much difference, the old Turbo SJs used a stock airbox.
