I think there’s actually two different questions here:
Would (if via sit-com magic) the Alex Keaton during the timeline of the show appear in the present, would he be a Trumpist?
and
If Alex Keaton’s character lived through the intervening years in our world to the present day, would he be a Trumpist?
I think the answers are probably slightly different, with a lot more unknowns for the second. For the first point, he’s almost certainly, after an amusing montage of trying to catch up on all the changes lo these many years, still a Republican. Heck, he’s probably just based on the changing times be considered pretty far right on a number of views. Since he does not suffer fools gladly, he probably dislike’s Trump, but fully envies what Trump can and does get away with. And Alex is far too self-centered to ever put Trump at the center of his world - he would, much like Trump, be all about what can Trump do for me?
Having said that, if (again, via sit-com magic) if he could get power/money/influence by being seen as a Trumpist, he’d be full on Trump-regalia all the time. So, I suspect he’s be a Mitch McConnell-ist more than anything else. Ride the wave for all he can get, handwave away the criticisms, admire what Trump gets away with while internally mocking Trump for being an idiot and imagining what he could do if he could channel all the idiots.
The second scenario is going to vary tremendously depending on what happened during the intervening years. If you go with the Spin City Canon, and he became a Senator, then he’s almost certainly in the midst of the Trump-excusing current mix of the Republican party. He’s still probably way too self centered to believe in that crap, especially after decades in the business, but he knows the risks of being critical, especially as Ohio leans further right. So it would be one of those ‘I wish he wouldn’t say the quiet parts out loud critics of Trump’ but blaming all the problems on the Democratic Party instead. He certainly wouldn’t vote for impeachment, or for a full investigation of 1/6, but he wouldn’t be trying to out-Trump Trump.
If we only stick to the end of Family Ties, we know he does well in school, and gets a job on Wall Street. Too many different things could have happened during that time: success, failure, getting rich, getting Madoxed, etc. My read - he made and makes a lot of money, but isn’t keeping much of it beyond appearances - mostly because he’s on his 2rd, 3rd, or maybe even 4th wife/lover. After all, he’s rich, he’s charming, and he is not particularly faithful. So the first few times, he didn’t have a pre-nup, and being found the offending party for likely affairs, got taken to the cleaners. He still does well, but the costs of supporting a few ex-wives, and possibly a current wife and lover are putting a huge strain on him.
In this scenario, he could actually be a real Trumpist to a degree. A casual philanderer, one who feels he’s been done wrong by ‘liberal’ assumptions of behavior, always being asked to not be a casual misogynist . . . there would be a lot of appeal. He’d probably say he was supporting Trump 100% for the typical Republican reasons - strong borders, lower taxes, more trickle down, less political activism, but deep down he wouldn’t be nearly as critical as the Senatorial version would have been.