Why is it that everyone thinks that Rodan 'saved' Godzilla? That makes no sense at all--they were bitter enemies at the beginning (and Godzilla brutally defeats him), and nothing else has changed since that time. Rodan has zero motivation to help Godzilla (who he doesn't even know is in the area since he was unconscious). At best it is coincidence that Rodan's demise revives Godzilla...though really, it's just contrivance at its worst.
Godzilla is dead, Rodan is killed and his body happens to fall onto Godzilla (how fortunate). Fire Rodan is the product of having absorbed so much of Godzilla's radioactivity from his ray as well as the radioactivity on Adanoa Island. Like the meltdown of Godzilla two films later, his body can't handle it any longer and he vaporizes into a radioactive mist. Lucky for Godzilla since radiation is about the only thing that can reactivate his regenerative ability. There's no transfer of 'spirit' or anything else to be read into it, it's just radioactivity. Just as Godzilla's meltdown revives Junior, Rodan's radioactivity revives Godzilla.
"If a boring person finds something boring, that is probably proof that it is interesting." - Akira Kurosawa