Horn has a fair amount less resistance to bending than wood has. It can simply be bent further.
Willow can bend pretty far. Willow is used to make cricket bats and is actually a wood with a funny set of properties.
Juniper and erc are nothing like willow. Incense cedar could work....
The easiest way to get a good core is to contact wood mills in Canada and get them to send you some green,flat sawn, perfectly straight grained maple. I've done it and got premium material. Indeed if I hadn't gone down this route I doubt I'd ever have got good enough material. The core must be absolutely flawless or else it will break.
Hornboiws were made with a set of materials that allow the thing to work, change one of the material to something subpar and you are in for a lot of heartache! By all means experiment but bear in mind that the materials settled on where the concensus of knowledge from many thousands of people who really knew what they were doing. A good craftsman would never waste time making something that may or may not hold together so they used solid, proven materials.
Anyway good luck