There's no reason to believe that the will itself is no longer valid. If it conformed to the requirements for execution back then, then it should still be valid. But, without the witness signatures being notarized and the witnesses being dead, it might be difficult to have it admitted.

However, while I'm not going to go searching, every state I know of has a statutory provision that a divorce invalidates any will provisions in favor of the ex-spouse. That includes bequests and nomination to act as executor.

Did this will contain any provision concerning your mother predeceasing him?