I guess, based on the 19.1 - 20.1 changes are mostly client-side, that the problem is not the registration, but that the client-side code of the widget is not working, probably because it’s not ported to TypeScript and therefore no longer considered an eT2 widget.
So I think your best bet is to start migrating the widgets to TypeScript
Unfortunately the process is not documented, though with our existing widgets you have many examples. There is a script which does some (not all!) of the necessary TypeScript migration:
Pull requests to extend the script or our documentation, are - as always - more then welcome