I spent a whole day trying to get Transmogrifier running under Plone 5.2 and Python 3. Looks as if nobody worked on this before.
The following branches contain basically the most relevant fixes for getting the modules into a Plone 5.2 without startup failures (which does not mean that the packages are fully ported or tested):
collective.transmogrifier = git firstname.lastname@example.org:collective/collective.transmogrifier.git branch=python3
plone.app.transmogrifier = git email@example.com:collective/plone.app.transmogrifier.git branch=python3
collective.jsonmigrator = git firstname.lastname@example.org:collective/collective.jsonmigrator.git branch=python3
I am not sure if I want to continue with transmogrifier in the current state (depends on further discussions with the customer about risk and efforts). But at least there is a starting point if someone wants to follow the transmogrifier road.