If you have loosened the cotter pin and it now slides freely back and forth for 3 or 4 mm, then the cotter pin is not stopping the clutch bell from coming off the crankshaft. The pin cannot move further than that, because of its shape. Try rotating the clutch bell on the shaft. It is almost certainly being kept from sliding off by dirt and rust on the shaft - wriggling it about may free it up without damaging anything. It can be removed by a gear puller of suitable size, if you have one, but that is a second-best option because the puller is capable of wrecking the parts - it can apply very large forces.
