If the lens is already crumbling, then I would smash out the lens & look inside the see what you have for fasteners. If no screws are visible from the outside, machine screws may be from the inside of the boat, facing out. Alternately, there may be some sort of catch that you will see once the lens is gone. You may have a screwed down base plat with a snap-on outer housing. Pictures of the existing hardware may help someone here to help you identify what you have.