If its an EI then perhaps you need a .csproj file and look at some of the examples, maybe its just a matter of adding this back:
11.2.300
You can also get any dlls from nuget and place those dependencies right inside your EI Folder and then just reference it in .csproj.
When Epicor detects a .csproj it will use it, if not it will just compile with basic libraries.