All you can really do is ask your developer for the project files, meaning any possible art, audio, lua, etc. files that there are. If you get the files, then someone else can pick up from where they left (of course it’ll take some time to get acquainted with the project, etc.)
Hopefully you have a solid contract that states that everything the developer creates while working on the project is your property, i.e. you already own the source code. If you don’t, then there isn’t really anything to be done. You can stick to the current developer or hire a new one. If you don’t have access to the source files, then the best you can do is provide the new developer the current build so that they can see what the other developer has approximately done, etc.