The current content of three existing SMF forums with an age-old version, all
of which share the same database, will only be made available to the users as
an archive.

I would reinstall the forum software SMF with the current version. That would
be php 7.x capable. The old content should only be accessible via an archive.

For the archive with the old entries I would put a link into the newly
installed forum.

1) Does that look like a workable way?2) Is WinHTTrack the right tool for me?