Hi, I just simply migrate character with default linked assets once, then migrate gamemode with default linked assets once, then the orange like level with default linked assets once, all of that choose override current files, it works, and I also copy the ini files from original project, as I can explain I think it works only because of those steps cover all of the missing files and links, if all codes are there, all works well, hope it will help.
And make sure you activate all the plugins as the sample project do.