For those of us who just cannot be parted from our beloved wooden cross country skis here’s how you replace your old 3-pin bindings with new bindings that you can actually buy boots for.
Step 1: Removing the old bindings
Unscrew the three screws from the binding (the third screw is hidden under the binding release in this photo),

and remove the cover on the heel plate in this case and unscrew that.

After which you will have a nice bare ski with the old screw holes in it.

Step 2: Plugging the old holes
Since this is a wooden ski and it probably didn’t cost very much (it may even have come out of someone’s trash…) we don’t really care that our ski has holes in it. However, in order to mount the new bindings properly we need to plug the holes with wood and glue (wooden matchsticks work well for this purpose) in case the holes for our new bindings end up near the old ones. Plus it’s a good idea to plug them anyway to keep the ski from absorbing moisture and getting woodworm.
This is the glue I like to use. It tends to foam as it cures which takes up any void space around the matchstick or the dowel, plus it sands well once hardened.

In this case I didn’t have any wooden matches handy so I used a wooden dowel which I had to shave down and sand to fit the holes. This is a little more work, but I don’t have much use for matches, so there you go. It helps to make the dowel a little tapered.
I filled the hole with the glue twisted the fitted tapered dowel in the first hole and notched the excess with a utility knife and twisted or snapped off the excess. A couple of technical taps with a wooden block and a hammer to snugly seat the dowel, and hey presto a plugged hole.

I’ll leave it overnight to harden and then I’ll remove the excess wood with a chisel and sand the tops of the dowels flush with the top of the skis.