tom23 I think the only options with existing extensions are:
- Use FoF Linguist to change the text and suggest to the user they could try logging in
- Use HTML Error Pages and Direct Links extension to replace 404 error page with custom content and a link to the login page
A custom extension will be needed to add a login button or redirect, and I don't think either exists at this time.
Adding a login button from an extension is currently very complex because the 404 error page is not rendered using Flarum's extensible frontend. Which means it first cannot be easily extended and second would not have access to the login modal component.