Is the bolt the only thing I need to change in a long action mauser for a magnum caliber such as 338 win mag

Do you mean a 8mm Mauser and you want to bump it to 338 WIn mag? If so the bolt face, feed rails , magazine box and follower and some rework on the action will have to take place. Just buy a FN mauser, CZ 550 or one of Mark X and use that. It will be a lot cheaper in the long run

was thinking of using a mark x style

The magazine box of a standard Mauser action such as those made for the -57mm series of cartridges is a little on the short side for cartridges with an o/a loaded length of .30/'06 dimensions or greater. In addition, they often need to have the rails opened to feed fatter cartridges of the diameter of the "short" magnums....