If you really were "co-authors" in a legal sense and agreed upon nothing, in my opinion you have joint copyright.
If you only were "co-authors" to be credited on Itch for your contribution and this project is mainly done by the other account, you are a contributor.
Removing your admin rights to the project changes nothing about this either way, so I seriously doubt that Itch staff could or would do anything about you having admin rights to a project not uploaded to your account.
And I have no idea how or if contributors have to be credited for a project. But I would assume that if you did not agree upon such a thing, there is no obligation nor right to have you listed on the presentation page of the project. Maybe you need to appear in the credits section of the game after downloading. Maybe not.
The fact that you seem to not be able to build the game from before "the fork", might indicate that you are a contributor.
But this is open community and not lawyer support. Do not expect qualified advise here. My suggestion is, find out what type of copyright you have, and how much it extends. Could you present your models on your own page as a prototype and claim that they were used in that other project? Could you force the other account to remove your contribution from the project? Could you force the other account to credit your contribution in some way? Could you rebuild the whole project from before the fork and have it on your account?
I use fork here, because that is what it sounded like. A software project where the developers parted and continued in different directions.