Fonts with Greek characters

I have an e-book which includes some formulae, embedded as images, and a number of individual Greek characters. The most common are sigma (σ) and rho (ρ). The text has apparently been scanned to create the e-book. because the isolated occurrences of sigma (referring to standard deviation) are rendered as the number 6. I would like to correct these instances to their proper Greek letter. That requires a font with a Greek alphabet, which I can expect to be present in common e-book formats like epub and mobi.

Do the underlying xhtml files of epub support Greek entities? What other possibilities exist?