I think Morphy without chess theory from the last 150 years gets crushed by most FMs and above. That's like sending the man armed with only a knife into a modern day battlefield.
Who would win, Paul Morphy or chess players in modern era?

Morphy's rating has been estimated (in modern terms) at about 2450. So there are hundreds of players nowadays who would beat him in a match.

Today there are many more chess players, particularly professional players, than there were in Morphy's time, so there are more players who are in Morphy's class in terms of chess talent. Naturally, these (few) players, with 150+ years of accumulated chess knowledge and their familiarity with Morphy's games (vs his inability to study their games) would be able to defeat him.
Whether lesser GMs, let alone IMs, could do the same is another matter. For the GMs, in a single game, probably. Over a match or tournament, he would see enough of the modern game to adjust his play and hold his own. Against IMs or lesser masters I feel Morphy would win. His innate ability would be too much greater than theirs. I have played a handful of IMs otb and never won (I did let a couple of drawn positions slip away) but looking at Morphy's games I admit I would be hopelessly outclassed.
Morphy didn't take chess seriously, only went to New York and later Europe to play chess because he graduated from law school at too young an age to be admitted to the bar in Louisiana, and convincingly defeated some of the world's strongest chess professionals, also astounding the chess world with his unprecedented blindfold and simul feats. It is ridiculous to think that just because he was unfamiliar with many openings or strategies used today that he would be unable to see what his opponents were trying to do and figure out an appropriate response. Remember that outclassed the other top players of his time by as great a margin as anyone else (Capablanca 1910-1920, Botvinnik 1940s, Fischer early 1970s) ever did.
This is quite hypothetical question. So, here are some assumptions we have to make.
1. Someone brought Paul Morphy alive at his best
2. Paul Morphy cannot access to the modern chess theories. (He will learn more during the match but we can ignore this)
3. Morphy's opponent is familiar with almost all of modern chess theories.
4. classical time control
For super GMs, it would be apparent that super GM will win the game because they will know more than Morphy and probably overwhelm Morphy in the opening.
In my opinion, even regular GMs will be able to beat Morphy because they probably know more opening lines, middle game tactics, etc than Morphy.
However, it is uncertain that whether FM or IM will be able to beat Morphy.
People who think FM/IM will be able to win against Morphy says that FM/IM will be stronger than Morphy because of modern theories and the help of AI.
On the other hands, some people think that Morphy will be able to win against FM/IM even with clear disadvantage due to his superb talent.
What is your opinion on this problem and why? Thanks in advance.