Thanks Fella's,
Here's a pic of the wheel with the little woodruff key in the centre positioning the sprocket on the crankshaft:
It looks like as if the circle in de middle is composed of different metal than both the inner circle and the cogs on the outside. The sprocket measures 2.5 inches diameter, and is less than 1 inch deep. The diameter of the crank is a little over 1 inch. My plan is to drill two holes of 5mm above the woodruff key, to create a wedge running from the centre to the cogs, to release the grip of the sprocket on the crank.
At the back of the sprocket there's one (now damaged
) of the two conical disks that guide the belt. Behind the disk is the oilpump, and the black object beneath is the oil sump