Define "clapped out"...? They're fairly robust, it's usually the cable that goes.
But if it is the speedo head that's had it, there are places that can refurb them and adjust them, if necessary. Do a search on yellowpages.com.au for "speedometer calibration" - I found a couple of hopeful leads there:
Advanced Auto Instrument Co of Revesby, and
Accurate Speedo Testing of Lidcombe
There were others as well.
CraigA has written up some good DIY instructions on this topic. See
here,
here, and
here.
cheers
RET
