Kenneth Regan's work on Intrinsic Performance Ratings includes estimated ratings for Morphy, which vary widely from event to event but average around 2300, which I think matches the intuitive perception of his strength that modern strong players have. https://cse.buffalo.edu/~regan/papers/pdf/Reg12IPRs.pdf

(Of course, as with all historical players, he would be stronger if he were re-animated today and exposed to modern principles and openings.)