As momo says, first remove the chain tensioner.
On the camshaft, there is the flat area where the sprocket normally sits (I.e. where the sprocket bolts up to the cam). Lets call this the large diametre.
You need to do the job with the sprocket off the large diametre by sliding it onto the smaller diameter section of your camshaft. This will allow you to get some slack on your chain. Wrap the chain around it, then gently slide the sprocket back on the large diameter (wrapped).
Ensure you have taken up most of the chains slack by gently rotating the crankshaft clockwise from the points shaft. This will give you more chain slack on the rear side of the motor.
With the sprocket back on its large diametre area of the camshaft, align your valve timing.
If you are doing this without removing the rocker shafts (it can be done). Do this with all your rockers fully undone (as far as they can be undone).
If your pins are out, just align the valve timing and bolt up the sprocket (using a little locktite on the bolts). Make sure you don't drop the bolts down into the case, so put a clean rag there to prevent that from happening.
Its a finiky job, stay cool headed and you'll be ok.
