Though a bit cumbersome, you can use
PDFRead 1.8 to convert those scanned .pdf ebooks into either the Sony .lrf format or Mobipocket .prc format (and then use calibre to convert the .prc to .epub).
There is no .epub support within PDFRead, but it can also be made to retain the .html and images used to create the resulting ebooks (using an empty file named "debug" in the PDFRead install directory) and thus that could easily be used with calibre or Sigil to yield a .epub ebook!
See the attached samples from "Japanese Fairy Tale Series 01 #01- Momotaro.pdf" noting that the .lrf and .prc were created by PDFRead using two successive "runs" and the .epub was created using calibre from that .prc.
It seems PDFRead is a good fit. Have fun converting all of them!