The original military stock would have had an oil finish, Linseed or Tung oil most likely. It is hard to tell from the pics you posted in the other thread, is it a cut down military stock or an aftermarket sporter one?
Depending on where the crack is, there are a couple ways to fix them. I have had a couple Mausers that the stock was cracked on the bottom, between the hole for the trigger and the hole for the magazine/floor plate here:
It is a pretty easy fix with epoxy and some threaded brass rod.
Best part is that it is hidden by the floor plate, can't tell it is there.