Alekhine should win just because he knows more, but potentially Morphy is a better chess player IMO.
"A popularly held theory about Paul Morphy is that if he returned to the chess world today and played our best contemporary players, he would come out the loser. Nothing is further from the truth. In a set match, Morphy would beat anybody alive today."
- Bobby Fischer
"He is known to have been familiar with such books as Bilguier's Handbook and Staunton's "The chess hand-book", among others.These books are better than modern ones; there has been no significant improvement since then in King Pawn openings, and Morphy's natural talents would be more than sufficient for him to vanquish the best twentieth century players."
- Bobby Fischer
"Morphy had the talent to beat any player of any era if given time to study modern theory and ideas."
- Bobby Fischer
Now I'm not qualified enough to criticize Fischer (same goes for everyone on this forum and most chess players), but I do feel he exaggerated or maybe Morphy was just that good. He did specifically say: in a "Set Match".
The magnificent American master had the most extraordinary brain that anybody has ever had for chess. Technique, strategy, tactics, knowledge which is inconceivable for us; all that was possessed by Morphy fifty-four years ago.
- Jose Raul Capablanca
Even Capa thinks Morphy is the GOAT in terms of pure chess talent.
You just have to enter a positional position and let him destroy his own position.
Or he'd get into some murky position where the engine says you're winning, but it's not easy.
You'd make a mistake, and he'd find the mate in 10