You won't be able to stop stuttering in time for your assembly speech. But you can create a shorter version, use words that you have fewer problems with, and memorise enough to trigger a sort of muscle memory. Practice breathing, and don't overthink it. It's a one-time thing and everyone will forget about it very quickly. And remember, even if you stutter when you speak, which you will or come close to, don't worry. Stop, recoup, and start again. No one's gonna pounce on you for saying it slowly. All the best, you've got this!