Sharon--
A Google search on "Mark Twain" and "Emperor Norton" turns up 14,900 items.
A glance at a few suggests that the belief that Norton is a model for the
King is widespread.  That certainly doesn't mean it's true.   But you would
surely find some of those items worth exploring.  Certainly Twain knew
Norton and spoke of him in print.  I imagine Edgar M.Branch's books on early
Clemens would be an excellent place to start serious research.

I have at hand a casual old nicely illustrated collection, MARK TWAIN'S SAN
FRANCISCO, ed. Bernard Taper.  It's fun, but also maddening because there's
no index.  But it does catch some of the most obvious Twain/Norton links.

The little I recall of Norton makes him seem an improbable model for the
King.  The King is a con man, generally ruthless in his motives, and fully
aware that he is not regal.    His kingship is merely a role  he takes on
for  a few weeks in his long life.  That doesn't sound to me much like the
good Emperor  (who also declared himself to be,  as I recall, "Protector of
Mexico").

Happy sleuthing,
Mark Coburn