If you use the oven cleaner trick, definately gonna have to refinish the wood. Tung oil or boiled lindseed oil.....
The muzzle check is simple. Place a military spec 30-06 (live) round into the muzzle of the barrel. The bullet will only go so far, and you look for the distance between the muzzle of the barrel and the case of the round. The more worn out the muzzle, the farther the bullet goes into the barrel, worst case, the case touches the end of the barrel.
Having said that, shoot your rifle first. The muzzle wear may not be that bad if your rifle shoots good groups. Barrels from the CMP run $60, and a gunsmith will mount it for you for a fee. Either way, enjoy your rifle.