Yes it's a strange title. It translates to 'The Ghosts of Omah-Ri.' The BFI has an entry, but it contains almost zero info, just the director and production company.
Also, it's listed in this Italian movie database as part of Richard Harrison's filmography, with 4 other spaghetti westerns Harrison did that same year.
It seems the director had a hard time getting his movies released at all. Una Forca per un Bastardo or Tara Poki aren't exactly well-circulated. Apparantly he also directed Karin Schubert's first movie, La facocera (1967), and again this is a lost movie. It's listed as unreleased somewhere.
So yes, looking for I Fantasmi is probably just a wild goose chase.