K31s are WONDERFUL rifles. You probably will never find another one crafted as well for 5x the price you paid.
Ammo uses standard .308 bullets and you can get the brass from Graf's:
www.grafs.com
Good luck making ammo MORE accurate than the GP11 (I've shot 1" groups at 100 yds. off the bench with factory iron sights with that surplus ammo), but the rifle will SHINE with a load it likes.
As for the stocks, they all look like that. I once saw a picture of a bunch of Swiss alpine troops at morning muster outside their pup tents standing at parade rest in 10" of snow on a mountain top somewhere. They had the butts of their rifles rested in the snow, against their boots. I always assumed the combination of snow and boot hobnails must have taken their toll on all those rifles.
As for cleaning them up, I did steam out a few dents on mine and went over the stock veeeeery lightly with fine sand paper (220-400 grit) just to smooth the surface, then applied a neutral oil finish. If you didn't see it before, you wouldn't know I'd touched it -- which is the point. If you don't have a light touch, and/or experience refinishing things without rounding edges and screwing up the wood-to-metal finish reveals, you'd probably be better off leaving it alone. The rifle will always be more valuable (and pleasing) in original form than if you've made detectable alteraions.
Nice buy, by the way! I paid $125 for mine about 6 years ago. Most of the time they go for well over twice that, now.
EDIT to add: You'll want to find some stripper clips, too. They are strange. Waxed cardboard with a metal lip. They look and feel flimsy and disposable, but they aren't ... and they work better than any other stripper clip I've ever used. Bizzare, and very cool!