Dukakis loses badly, as in real life. That is, unless Reagan's health issues are leaked.
There was nothing to be leaked. He didn't start showing signs until 1991.
To be fair, he already started showing signs of dementia well into his second term.
What's interesting about this scenario is where that leaves Bush for 1992. Could he have beaten Clinton (assuming Clinton would run)?
Actually, when Howard Baker when in as Chief of Staff (1987) there were concerns among the staff that Reagan was not up to par. After one meeting him, Baker said Reagan was in full control. He would have been reelected by a landslide in 1988, though it would have looked more like 1980 than 1984.
