One of several good places to start might be the
Wikipedia article on the Suzuki Cultus, the progentitor of the Metro, Pontiac Firefly, Suzuki Swift, et al.
This has links to the
GM M Platform as well as the
Suzuki G Family of Engines.
Like other Wikipedia articles, its up to you to vet the information contained there, but these articles happen to cite numerous verifiable sources.
For a 'living' source of information, by all means head to
TeamSWIFT.net -- this is the most active and current forum dedicated to the Metro and its siblings.