Swim ladders are available from many vendors and in many styles. My expectation is any swim ladder or boarding ladder previously installed was an off-the-shelf commercial ladder, and not something fabricated by Boston Whaler specifically for that year and model boat.
On many Boston Whaler boats the boarding ladder is mounted to a small platform installed on the transom and supported with a custom-bent stainless steel tube which also functions as a hand railing and aids the swimmer in climbing aboard. If that platform is not on the boat, then you will have to create something similar for mounting a stock swim ladder.
This newer Outrage has a boarding ladder mounted to a small platform on the port side of the transom.