My billets are absolutely disgusting and they're grey rather than black. It's a used saddle but everything else is in great condition. It came like that and no amount of soap seems to help it. It's flecking and cracking and just not in good condition at all.
Is there a way to replace the billets on my saddle or am I stuck with what I got?
Good news and bad news. Good news is, yes, your billets can be replaced. Bad news is, you shouldn't do it yourself, and in most cases won't be able to. You will need to send it to a saddlery to have them replaced.
If it's only cosmetic, I wouldn't worry too much as when you are riding, they won't be seen. If the leather, however, is what is in really bad condition, I would replace them.
Be careful with trying to soap and clean them too much, as this will only make it worse. I actually cheat and put a leather dye on mine that our local saddle shop suggested to make mine look better.
If it's stiff and cracking, I would try conditioning it a little, just to get it more flexible, but I would be careful because if they are in really rough condition, they could break during a ride, and that is very unsafe. Check them before every ride to make sure none of the cracks are going completely through to the other side.