Would you recognise Napoleon or Washington if you found them on an 18th century sleeper ship, with their soldiers, in their bright garb and funny wigs?
I might entertain the notion that there was somebody historically important aboard and note that their leader bore a suspicious resemblance to that guy on the dollar bill.
And that short, imperious French commander from the 18th century, the one who looks like the guy in all the paintings and Napoleon movies? Hmm, I wonder . ...
I mean, sure, I wouldn't have a clue what Patrick Henry or John Quincy Adams looked like, but Benjamin Franklin or Mark Twain or Robert E. Lee? I'd know what they were supposed to look like at least.
(I once spotted an off-duty Franklin impersonator hanging out and drinking a Coke at a Philadelphia train station. Not for a second did I wonder who he was supposed to be.)
And remember, Marla isn't relying on old, unreliable paintings and statues. There's presumably no shortage of photos and film footage of Khan, despite the war. The same way we have actual photos of Civil War generals and politicians.
A lot depends on your particular fields of expertise, of course. Would I recognize Disraeli or Henry Clay if I saw them in the right context? Probably not. Mary Shelley, or Bram Stoker, or Edgar Alan Poe . . . absolutely.