Preserving the original leather sling is a great idea because the old dry leather will continue to crack and get damaged if you neglect it. Using a repro sling is great on a Krag but having an original sling on a collectible piece is even better.

There are many kinds of leather treatment waxes and balms available. My favorite (certainly not the only one) that I have used for years on leather boots, shoes, holsters, belts and slings is "Sno-Seal". It has beeswax and  other preservatives in it that soak into dry leather well and recondition it to look great. Nothing will heal a deep crack but something like this treatment will fill smaller ones and provide a moisture and lubricant that will keep it from further damage.

Look around and find what might work for you. The leather looks so nice when you recondition it. Use it liberally and let it soak in a few days before wiping the excess off. It works really well.