Yeah the pulley is keyed.
Best way to get at the pulley is to pop the back wheel off then you should be able to just get to the bolt from within thw wheel well, just under the chassis rail.
If you're lucky you'll be able to get away with whacking the handbrake on tight and sticking it in gear to stop the engine turning over whilst you undo the bolt.
Next option would be to have an assistant pressing hard on the footbrake.
Last resort is undoing the flywheel cover on the underside of the engine/gearbox and wedging a big screwdriver into the teeth of the flywheel.
Once you've got the bolt undone your next problem is getting the pulley off.
You've got one advantage over most people in that yours is already knackered
(most people doing this for the first time will try and pry it off but being a cast piece the lip on the edge of the pulley will chip very easily doing this.
) The correct approach is to use a puller of some kind, if you dont have one then you can fashion your own tool to do the job.
Basically you want a short length of quite sturdy metal which you can drill and bolt to the pulley via the two holes in the centre of it.
If you then place a suitable sized socket between your tool and the end of the crank then the action of tightening the bolts will pull the pulley off the crank.
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