as far as i recall, 6300 is a Series40 phone and i do recall that trying to delete built-in themes won't be allowed as the themes are protected files. there is no way to remove this protection, so you will just have to live with the fact the phone memory is very low. my suggestion would be to leave the themes as they are, and buy a microSD memory card to store your other data on.
if you restored your factory settings and those options are missing then maybe something is wrong with your phone (providing that the email options and the calculator are pre-installed and part of the phone's firmware). so go and take your phone to your local nokia care centre for them to have a look at. however, if they are applications that you downloaded and installed yourself, then of course the phone memory will be wiped, this happens with all restoration of factory settings/firmware updates.