Actually, Wilson was hoping he would be renominated for a third term despite the stroke. He seriously thought about seeking a third term because he was absolutely (and stubbornly) determined to get the League of Nations passed. So to have Wilson running in 1920 is not as far fetch as some may think though I bet Thomas Marshall is Wilson's running mate.
As for TR, that is also not far fetched. By early 1919, the Republican party was rallied behind TR from the conservative politicians like Taft and more progressive Republicans like Hiram Johnson. Seriously, only his death prevented TR from running for president in 1920 and receiving the nomination.
As for the election, it would have been a landslide for TR. Wilson got lucky twice in his victories. In 1912, it was a split in the GOP that got Wilson elected and in 1916, it was Hughes that shot himself in the foot by not meeting with California Governor Hiram Johnson costing Hughes the election. This time nothing would save Wilson from defeat.
Roosevelt: 417
Wilson: 114
TR probably gets around 65% of the national vote.
