If it's just the sole, you can cut the stitches carefully and use the old one as a pattern for a new sole. The hardest part is getting the holes on the new soles to match up as closely as possible.
I don't know how particular you want to be on matching the leather, but it will be almost impossible to find a perfect match. I've seen re-soled mocs with a darker leather sole and I thought it looked nice.
One thing I want to try to find is some rubber sole plates to attach to the 2 pair of Minnetonka mocs I found at resale stores. Even with the double inserts, I still have to be careful where I step to prevent rock bruises. I wear through mocs quickly. A little extra expense to help these bargain finds last longer and provide additional protection for my feet is worth it.