You should not fork the code, especially for some small little thing that can be easily done without forking the code. You are creating difficulties for yourself doing that, making it harder to upgrade to new versions and adding unneccesary maintainenance tasks to keep your customization.
1. copy and rename the mojoportal.sln file as discussed in the article Setting Up Your Projects
2. add a class library to your custom .sln file
3. add a reference in your class library to the mojoPortal.Business and mojoPortal.Business.WebHelpers projects
4. add a class to your class library and copy in the code from the DoNothingUserSignInHandler, but rename the class and the namespace. Now this class library can be compiled into its own dll which can be put in the /bin folder of mojoPortal.Web project
5. finally make a custom config file that references your namespace and class as indicated in the article User Sign In Event Handlers, and put that file under Web/Setup/ProviderConfig/usersigninhandlers
If you go down the path of forking the code you will later regret it when you need bug fixes or new features.