What an intriguing melody.

Here's a solo guitar version

My Spanish is limited but it looks as if you can get a hold of Senor Sosa he may send you a guitar transcription of the piece. At the very least you might be able to pick out the chords and notes from his playing. He's capoed II and playing in F#m which is the key that Theodorakis plays it in the link above. Fureys do it in Em judging from the link to their version.