Hey Simon,
You know those are for shooting bears, not letting them play with. ;~)
Very cool old piece of bow history. Can't wait to see what you do to it.
I'm not a bowyer, so I don't know squat about bow restoration. I do know something about restoring antique firearm, furniture, knives, that sort of stuff. It's hard to tell in pictures, but a lot of that damage looks a more like a gouge or dents than missing wood.
If it is, a damp wash cloth and an iron may pull a lot of it back out. You may have already thought about this, but I thought I'd mention it anyway for those who haven't tried it. I think I might try it first, before I did anything else and see if it won't pull some of it out. It will at least raise the grain a little and leave you with less fill material to put in.
Dampen a wash cloth and fold it in half. Lay it over the dent and iron it on medium heat for 20 or 30 seconds to start with and see what happens. It may take a few times to lift the grain as much as it will raise.
You may have to raise the heat a little depending on the iron, but you want it to be hot enough to lightly steam the moisture in the cloth. But not make it sizzle! Be careful not to use more heat than it takes to steam it. Too much heat for too long can damage the wood and actually set a dent so it can't be raised.
The whole point is to use the heat and moisture to soften, hydrate and swell the cellular structure of the wood. I've done it dozens of times with varying degrees of success. Some gouges I thought would never come out just disappeared. And some of the ones I thought would be a snap wouldn't move a bit. But it's worth a try for sure.
Hope it works. And as I said, can't wait to see how it comes out. Be sure to keep us updated. That's a cool old bow.