Thank you! For some reason I thought Foster was part of the Celeste64 build.
Building Foster, placing it where Celeste64 is looking for it, then downloading the FMOD libraries from
https://www.fmod.com/download
FMOD Engine > Linux; requires an account but the account is free iirc
and placing those where Celeste64 is looking for them, and it works via dotnet run! (Or at least it launches, I will have to wait a bit longer to try playing it.)