You mention that the pin doesn’t appear to be the stock roll pin. Is it rusted up? It is possible that it is rusted and disintegrate over time leaving the seal assembly inoperative. Like Frank says, pull the upper driveshaft out of the unit with the impeller housing/impeller, then file the ridge on the bottom of the driveshaft. You may have better luck removing whatever is being used for the roll pin by clamping the upper driveshaft in a vice, using it to drive the roll pin at least flush on one side, then work it back and forth until it can be driven out with a little punch. On the other hand, maybe you want to leave it alone if it is not rusty, who knows what someone might have done/used instead of the OEM roll pin. Post pictures if you are concerned.
