the type of locking pin you have is bolted into the aluminum sheave. The hole in the aluminum is tapped and the stainless steel unit is threaded. It is originally mounted with hard LOCTITE thread compound, and a nylon washer on top of the aluminum.
Ther is no nut at the backside of the pulley. You will certainly need to heat the area of the threaded part. You should use a small portable LPG welding tool in order not to heat the pulley too much (and damage the paint around).
Once removed, you should be able to drill a new hole for the pin. Try to make it 1 mm wider than the pin, and put some silicone grease or some sticky anti-corrosion stuff to avoid stainless steel from damaging the aluminum drive plate (the new hole will not be anodized).