Hi, who do you have to do the presentation in front of? Have you asked if you could do it in front of just the professor, now that he knows about your stammer anyway you might feel more relaxed. People WITHOUT stammers get nervous during presentations and fumble their words sometimes, and I recently did a presentation and remembering this fact helped me. Just remember to breathe and speak from your diaphragm. In my opinion, you will feel much better for having completed a presentation despite the obstacle rather than skipping. Your grade should not be affected by stammering if you have informed the professor. :)