I would be most surprised if the original cantles were sand-cast. They look to me to be constructed (jeweler's term for saw and solder work as opposed to casting). So I think you are making yourself a bigger hurdle than you need to here.

Just a thought: I've wondered if modern attachment schemes might be brought to bear on the leather work, so that one could swap a hair sporran for a leather one on the same cantle? Velcro?