Have had experience with helping a couple of users get off a Moodle Partner site to a locally hosted 'plain jane' Moodle before.
MP's gave former customer db dump and moodledata backups but not the code - obvious reasons.
So I had set up a test site using git acquired core code with the DB dump of the MP (as close to the version of core as possible) and the moodledata from MP.
Could login as an admin level as those accounts were manual and then had to visit plugins to see which were missing from disk. Removed them.
No magic bullets/code exist to reverse engineer me thinks, just a lot of sluething.
IF it's just authentication, one could directly manipulate mdl_users table changing auth column (password reset?).
Hopefully a student could login but then there would probably be issues with enrollments to courses. More sluething ... and deep I would imagine.
Tutora's 'claim to fame fork' is multi-tenacy and better reporting so those would be areas to concentrate upon in converting. Sorry if that is 'Captain Obvious' but ....