Well, apparently he did suffer a minor stroke at the conference, which caused a twitch of his facial muscles. There's no 25th Amendment, but SoS Lansing and VP Marshall would have to convince the Congressional leaders to allow Marshall to become Acting President. If Wilson still refuses to resign, they'd have to impeach him on grounds of medical disability. Marshall was repeatedly asked to become Acting President IOTL, but refused to do so. For the last two years of his term Edith Wilson and the Cabinet ran the country, far more than any Dem ever dreamed Nancy Reagan did.