Put this in perspective. Your talking about an older aluminum boat. How long did the original transom last and what was the real cause of failure? Was it the fact that it was made out of wood? Hardly. The real problem was that it was allowed to get wet. Who ever installed it originally, or who ever drilled into it to install the motor or accessories, didn't properly seal it.
Wood is still the strongest material available. Some of the later model GLASS boats that use a non-wood core. make up for the strength factor by using additional layers of glass. You don't have that option with an older aluminum boat.
Here's a wood product that has a lifetime warranty against rot:
http://www.greenwoodproducts.com/xlpanels.htm Many of the top builders are using that product in their boats today.
If you don't use the product I mentioned about, use marine grade plywood.
Regarding the seacast product, in order to use it, you need to have "pocket" to retain it. Many older aluminum boats do not have an inside liner or skin on the inboard side of the transom, and if same applies to your boat, your going to need to fabricate something to retain (hold in place) the poured seacast while it sets up. But why bother. Go with wood.