If it was my boat I would not replace it with a piece of metal. I would cut a shaping form and then buy carbon fibre and fibreglass tape. I would laminate these alternatively until I had a solid beam that was thick enough. It would not rot and it would be very easy to install if I had the foresight to lay in some cross strips to use as tabbing from a few different layers.
Failing that I would do as SD suggested. The last thing I would do would be to put metal in my bilge area.
GOod Luck
