Mate it would be a rare day that a wet starter motor has to be thrown away. Strip it and clean it up, shout it a set of new carbon brushes as they are dirt cheap then assemble and bolt it up. Make sure the bearings are in good shape and lubricated or replace them, again nanother cheap part.
Having said that I noticed that someone either on Fleabay or FaceAche advertised aftermarket starters for Listers. In this case it was for a SR1 but they do make them for multi cylinder Listers. A Google search or search on Fleabay should get results. You will need to mention if yours is clockwise or anticlockwise direction.
Glad your back on the water and not under it.