We used to use PB Blaster, but get better results from, Sea Foam Deep Creep.

I would give the sheath a gentle scrub with baby shampoo and warm water. Let is fully air dry then rub in some layers of Obenauf's Heavy Duty LP Leather Conditioner Natural Oil Beeswax Formula.

Cool knife, good luck with the restoration.
_________________________
Brian - Michigan-NRA Endowment Member
"A fear of weapons is a sign of retarded sexual and emotional maturity."
- Sigmund Freud