Yes the headshell is the missing part, and there are plenty of standard models to fit into this arm. The simplest thing would be to buy the standard Technics headshell, which is readily available (http://www.needledoctor.com/Technics-Headshell
, for example). You might find it cheaper elsewhere - on Amazon, for a start.
You would also need a cartridge, which bolts into the headshell and carries the stylus/needle that actually tracks the groove. There are a few packages of headshell and stylus, including special integrated one-piece models that just plug into the end of the arm, but most of these are for DJ/scratching service, not hi fi.
So, instead, how about fitting an Audio Technica AT95E (http://www.amazon.com/Audio-Technica-At95e-Mm-Cartridge/dp/B001MRXY94
)? This would bolt easily into the headshell, then you just need to connect the wires to the back of the cartridge, and position it in the headshell (see alignment guides on this site). The arm is calibrated, so you just need to balance it, set the tracking force and anti-skate, and you'll soon be listening to your LPs again