Elo rating of the best player from the 19th century, if he played today? Take the best player from the 19th century, at his prime, and imagine he started playing tournaments today. He can't learn new stuff.
What Elo would he achieve?
Ken Regan's research (e.g., Understanding Distributions of Chess Performances - PDF link) suggests that the best players of the 19th century were playing at a level in the 2400-2500 range (see the table on page 9 of that paper).