I've done several of these, and interestingly enough I am in the process of putting together a GPR flintlock kit, and I did exactly as you, and when I was finished I didn't like the toe plate that came with the kit so built me a new one for some sheet metal I have. I have also built toe plates out of horn before, looks really nice on the rifle it is installed on, but it was a bit difficult to get the piece to lay flat so I could shape the desired size needed. I am no expert but what I do is draw a line around the item to be inlet, such as your toe plate, and I use the sharpest lead I can come up with and keep the tip at an angle so that the line is right flush with the edge of the toe plate. Once I have my line then I use some very narrow chisels to cut the curved portion of the plate. In the past I have designed the toe plate to be slightly larger than the width of the bottom flat of the stock, that way once it is installed I can file it back to mate up smoothly with the wood. I once tried to use my dermal tool with a small router bit chucked up in it to cut the forward rounded end of the plate, only did this once, learned that it is much wiser to use small chisels and take my time. In the past I made my own chisels for this type of delicate work using either a good quality screwdriver that would sharpen and hold an edge, which I found most won't, so I started making them out of allen wrenches, or small easy out screw removers, they are usually made from fairly good steel, and simply heated them up until they were soft and forged them to the size and shape of edge I wanted, then rehardened and tempered them. A couple I have out there I have been using for years and they can be sharpened to a razor edge and hold that edge for a good while. One is about an eighth of an inch wide the other is smaller. With those two I can make some pretty tight curves by simply working my way around my layout lines, going slow and easy. There are times when once you redo a job like you are talking about the original holes in the wood won't mate up with the holes in the toe plate, if this happens you can simply plug the original holes with dowals or match sticks, what ever fits in the hole good and tight, and I simply use Elmers wood glue, once they are set and the glue has cured I mark and drill the new holes, never had a screw pull out if I did a good plug job. Hope this helps, and remember, if you cut out too much wood you can always get some sheet steel or brass and make a longer toe plate, due to their simple design they are not all that difficult to create. I purchased a brass door kick plate several years ago and then never used it, since then I've used that to make a number of inlays out of as well as a backing plate for my porch light when I could find nothing else that would work, it is fairly decent brass and not too expensive, not as thick as the original steel toe plate, but then toe plates don't have to be all that thick really, at least not in my opinion. Good luck.
