One of the more challenging parts of building a radio from scratch is fabricating something to turn the radio signals into sound, namely an earphone or speaker. Everything else is quite simple, as long as basic technology to make metal is available.
On my blog, I describe a method to make piezoelectric crystals, and turn them into a headphone. This could all be done using off-the-shelf supplies using post-1675 technology. Or, with a bit of additional chemistry, it could be done with even less technology. You can read more at this link:
http://onetuberadio.com/2015/12/06/piez ... tic-needs/