Hello, I'm back!
snapcraft is flexible enough to let you do (almost) anything. So you could use a released tar.gz as the source without problems. However, that's not the approach I would use.
Now that everybody seems to be happy with the quality of the snap, and @roxd explored the features available in snapcraft, I would recommend to contact upstream to see if they are interested in maintaining it. If so, after the transfer they can just hook their CI system to make continuous delivery following the instructions here: https://snapcraft.io/docs/build-snaps/ci-integration
Whenever they make a new stable release, that will get immediately to the store and can be made available to all the users in the stable channel.
If they are not interested in maintaining it, this snap repo can also be hooked to the daily cron from travis, so it's extremely low maintenance for roxd and she can use her time in something more interesting like new snaps or other types of contributions.
I think there's no need for a different snapcraft.yaml here. And I'd be happy to help in any of the two cases. Let me know how it goes.