I've actually done exactly that many years ago for a self-hosted Jira installation that didn't have any backups. You can bet we had backups with regular testing after that.
Honestly it doesn't sound too difficult and like a challenge to script something fun. To me. If you ever find yourself in that situation, email is in my profile and we'll work something out :)
In my limited experience these diffs can be missing information. I recently had to reconstruct an issue description using these email diffs after two people where editing the description at the same time and it was not 100% accurate, several lines were missing. Going to the 'history' tab on the issue I was able to get the missing lines however, if all you have are emails though you might be out of luck.