Blank message body in Mail for old .emlx files, but content visible in Get Info preview

I have a lot of old (20 years) email saved as .emlx files. When I try to open them in Mail, the message body appears as a blank white space. However, when I go to Get Info on the file and choose Preview, I can see the text of the email within that window within the Get Info box. Any suggestions on how I can get the content to appear in the actual Mail application?

[Re-Titled by Moderator]

Original Title: Old .emlx files appear as blank messages in Mail, but can see content in Get Info window