Go for what you already have. Use your supervisor to cut and twist your proposal plan to fit 5 months. I don't know where in this world you are, but where I've taught, a master's thesis is about demonstrating what you can do in the time given towards a relevant result. A proposal is a plan. Relevant mileposts should be reached, and the final goal to some extend. But details can be adjusted and cut during the process. A supervisor should help with adjusting the project plan. Up front as well as during your project.