One method is to find a bit of tube with the inside diameter matching the outside dai of the piston and drill a pin sized hole right through the centre of it. then sit the piston in it rotated to line the pin up with the holes and use a piece of bar ever so slightly smaller than the pin to knock it through. serious care needs to be taken with the whole thing, its a massive ball ache. hitting the end of the rod with a torch can help to loosen the pin too, but its not amazing for the structure of the metal.