Yes, you can remove the pan without removing the engine. Loosen the pan bolts and let it drop as far as it will go so that you can access the cotter pin holding the oil pump float in place. Remove the pin to allow the float to drop into the pan. Turn the wheels so the center link is out of the way and you should be able to slide the pan out. You'll also need to remove the clutch inspection cover. Sometimes, it may be necessary to lift the engine an inch or two to make this work. But, the engine doesn't have to come out.
That said, if the oil didn't have water in it and only appeared dirty, I would opt for just cleaning out as much sludge as you can thru the drain hole. Get some good carb spray and shoot some in the drain hole and see what comes out. Oils used back in the day had a lot of paraffin in it and as a non-detergent mix, tended to sludge more than modern oils that suspend the particulates for the filter to trap. If there isn't much in the pan, add fresh oil and see what happens.