With the understanding that I am not recommending or suggesting that rifles deactivated that way can or should be restored to firing condition, I think it could be done. It looks like the receiver and barrel tang were drilled through and (probably) a steel pin inserted and the head welded in. The weld and pin would have to be drilled or cut out, using a hard drill, then the barrel removed and a new barrel installed. If the lug seats have been damaged, they could be welded up and remachined. A ruined bolt could be replaced. Then the holes in the receiver could be welded up and the welds ground to surface.
Would the result be safe to fire? I think so, at least for a limited amount of firing; the Mauser 98 action, like most guns of that period, is way over-engineered.
Would it be worth it? No, from two angles. First, the cost would be so high as to make it infeasible, even if possible. Second, countries that deactivate guns generally have severe, even draconian, penalties for attempting to reactivate them or for possessing an operable gun. In some countries, gun law violators can face life in prison or even summary execution. Revolutionaries may be willing to accept the danger, but it isn't worth it simply to have an operable gun.
Jim