M98s are typically "large ring" actions (not always, there are some small ring M98s, but that's another issue). Mauser 1896 rifles are "small ring" actions. This means that the front of the receiver is actually physically larger on the M98s, so the stock has material removed in that area to allow the action to fit into the stock. Also, the guard screws are slightly farther apart on a M98 than on a 1896 (about 3/16" farther apart) and the magazine box is physically longer. Long story short, they won't fit. Depending on what synthetic stock you wanted to start with, it could be done, but you would be better off just buying the correct stock to begin with.
