The only reliable firm I've heard of for motorcycle (as opposed to moped) rentals is this:
http://www.motorent.gr/ and they sound expensive. Check them out though.
BTW, I have checked a couple of firms who offer guided bike tours of Greece including one who uses older T bikes. All their tours are oriented towards the typical tourist destinations and stops (i.e. ancient sites) and none have true biking and seeing the real rural Greece as a priority. Too much motorway riding also. Might be a good idea if you're interested in such a 'safe' thing though.
