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.

Teachers have the resources to check for plagiarism and would be able to check for such things. I would recommend checking the project through one of these resources and let them know if you could spot it, so can your teacher. Then, you protect your reputation and put the responsibility back on them. So that once your teacher spots it, you will have proof you let them know in advance how you felt about the submission.