Many of the customs in that thread are G1 to G4, not G4 to G1. I have seen a handful of G1 FiM customs around, so hopefully others will post. The G4 characters are so stylized that as a customizer I personally haven't been tempted to do this because the pony personalities don't seem to fit the more sedate body style of G1s. Just a taste thing on my part, but it's a fun idea to try!