ANDYW,
Dall sheep have hair that is hollow core, just like most cold weather ruminant animals such as caribou. The problem with this is that the hair, especially when its subjected to sunlight and age becomes very brittle and breaks down. The two worst things for a for are smoke and sunlight. When you touch the mount does hair come off the mount, even in small amounts? If it does the chance of having it re-whitened is probably not a good idea as it will most likely make the hair fall out even more so with the chemicals from a whitening job. The horns wouldn't be a problem to restore, but the hair, due the sunlight its been subjected too AND the age of the mount may not be an option. I use a mount cleaner product on previously mounted animals, however this only removes some dirt and smoke color, but a true whitening job on the Dall may be a real risky venture.