I believe my classmates plan to hand in a plagiarized project, how should I handle this?
Should I inform the project teacher, to have them exposed to the exam committee?


Yes. Tell the teacher exactly what you know: that there is a game that seems very similar to your classmates' project.

As a student, it's not your responsibility to decide what is and isn't plagiarism, or to decide who should be punished for dishonesty and who shouldn't, or to start a "movement" to let the cheaters know you're onto them. If you believe you have evidence of misconduct, report it and let the administration handle it from there.