I'd press the fancy molding into a square openable mold that has FIMO or other plastic modeling materials in the bottom and bake them. Then grease the mold and make the castings.

Spraying the carving liberally with cheap furniture polish prevents sticking.

+1 on the Smooth-on product line. The silicone rubber molds and urethane casting products make essentially perfect reproductions and it's actually a lot of fun to do. $100 in supplies seems like a reasonable starting point, but should be enough to allow you to make a number of smaller molds, no just one.

Abatron makes a silicone-rubber mold-making system, and you can use their pourable "casting epoxy" (Abocast, IIRC) or Bondo, or Plaster of Paris. I have used the Abatron system a few times and it does work pretty well.
Casey

Jennifer:

My problem with your idea is the removal of original fabric to replace with reproduction. I doubt your changes are easily reversable. If the original builder of your house wanted more detail, he would have built it. Be a steward of his vision, please.

Joseph ... I saw nothing in the original request that indicated the OP was going to rip out the animals and replace them with flowers. Maybe she wants to add trim in a room where it's been removed.

If we were the "stewards of the vision" of old houses, we'd be living with oil lamps, wood burning stoves and an outhouse in the back.

You don't need to substantially remove decorative original fabric to install electricity, heating, or plumbing.

Joseph, I do not want to remove anything in my house. I merely want to make duplicates of what I have. I have some doorways in my house that were closed up and walled over for decades, and now that they are open again, it would be nice to have matching corner and plinth blocks around each window/door in each room. I can't just drive to the local big box store and get some blocks to match my original ones.

As for the "flowering dogwood branch" blocks I picked up at the estate sale - I bought everything that they had, which amounted to only 2 blocks. I'd like to have more of these to redo a bedroom at my parents' farmhouse. Maybe the flower blocks are too fancy for a farmhouse, but I think they are lovely and would look great in that setting. My problem is, I would need more than the 2 blocks I currently have.

Just blundered into this:

http://www.townsendatelier.com/store/knead_a_mold.php

It looks easy enough

We made a mold to replace the braid/lambstongue molding that is fitted around all the modillions in this cornice. we cast them in good ol' bondo. the originals were compo.

Sombreuil, what did you make the mold out of? Silicon?

I've used bondo to repair/patch woodwork before..

It was the Abatron mold-making kit, which is a 2-part silicone. I wanted to point out that it takes a certain level of intricacy in a design before it becomes practical to bother with mold-making. The OP's slightly spoon-carved/incised corner blocks are a woodworking project; I just can't see making them from synthetic material,\, when they should be wood; they need to have a consistent thickness, and be flat and square, none of which can be achieved by pouring resin into a mold.
Casey