Looks like the original plaster covered the wooden base, which makes for excellent repair possibilities. Though removing the existing base may prove tricky.
As a result you may not want to remove it at all. As Fixer1234 points out you may run into some additional problems if any of the armature is under tension from how it was attached to the wood, something we won't know until it either has or hasn't caused additional damage on removal of the base. As a result you could leave the base in place, and apply a wood hardener, which come in many brands but are resins and epoxies designed for filling in the damaged parts of wood making it firm again. Since the wood itself is not part of the visible statue, you could save a lot of trouble simply by leaving it in place and letting the plaster restoration cover it.
In the situation that the wood is really crumbling away, as in you can peel it off with your fingers, the wood hardener won't work very well. Though at that point removing it should be pretty easy.
In all of the below possibilities, use hand tools. The vibration of a power drill or other tool might help shake the base loose, but it is more likely to do damage to the statue. Additionally you may want to trace the outline of the original base before pulling it off, in case it is destroyed in removing it.
After she is completely dry. I would lay her on something soft and look underneath the wooden base to see if there is any obvious attachment to the rest of the statue, you may find that an armature has been bolted to the wooden base and goes through the plaster statue to help support it from falling under its own weight.
If you can find a bolt remove it. Then give the base a bit of a jiggle see if it just comes loose. Not much, just enough to shake loose any plaster touching the wood. It is quite possible that even if there is a bolt, it is not the only thing holding the armature to the base. So...
If you don't see anything bolted, or the jiggle didn't pop off the base. The armature likely is many smaller, still heavy, wires that fit into holes drilled into the wooden base, and probably glued into place. For 20 pounds at 49 inches I would guess that she is hallow. Which is useful as you can probably get an idea of where the armature is attached to the base if you can see inside. Looks like the damage between her feet might offer a place to look. If you can identify any attachments to the base you may have some luck pulling the wood out from behind them. If it is really rotten wood it will come away quickly, and even mostly ok wood can be removed with careful scraping from some wood carving tools, such as a gouge or a small chisel.
If you're not finding how the base is attached you might want to trim off bits of it in layers. If the wood is significantly rotten, it will fall away by itself. Otherwise either without a hammer, or with the lightest tap of a hammer put the chisel at parallel with the grain of the wood near the bottom and twist it a little bit to pull a chip off. It would be slow, but you could potentially pull the whole base off in chips. I would only really do this if the wood is rotten to the point that it is easy to do, if it is still pretty hard, I would get impatient and rely too much on the hammer.
Once you have the base off, a new one will be fairly straight forward. Take your tracing, or the original base, and place it on a new piece of wood of the right thickness. Trace out the oval shape. You can cut it out with a hand saw, just cut out corners until it sufficiently resembles an oval. If you're comfortable working with them, a table saw or jigsaw would work as well.
If there were holes drilled in the original for the armature to fit in, put some chalk on the bottom of each armature wire and bump the fresh base into them so there is a chalk mark for where to drill the holes. If there was a larger rod bolted through the base, drill the hole for this first (can use the chalk method here too) then go back for the smaller wires if they are also present such that the base can now reach them without getting caught on the main support.
The best part about it, is if the cut for the new base isn't perfect, it will be covered by plaster when you're done, so no one will see it. As such you shouldn't need to make a jig or similar to cut it out, by eye should be fine.
