Ideally they should be the same size - especially the diameter and the distance between the screws on the bottom so that they fit into the original board - and specs. Hard to find them... I'm not sure that the Cornell Dubilier in the link above would work but thanks M Code.
Gemstone hasn't replied yet, probably never will. I did contact Sim Audio, they made the amp for Gemstone, but they were not willing to help at all...
Still looking, wonder if anyone knows what the 'A' on the capacitor stands for i.e what company that might be?