I've seen 2 different manufacturers make synthetic for leverguns. Only for Marlin and Winchester 94's. One was a cheap piece of junk, the other cost about 2X what the rifle cost. I'd just keep the wood on a levergun.
I did read a post from a guy who lived in Alaska a few years ago. He painted his stainless guide gun stock with the stuff used for spray in truck bed liners. Looked ugly as sin, but tough as nails. Just a thought.