A single week isn't going to really put much of a dent in your running fitness. I think it's better to keep the impact load pretty light and give you ankle proper time to heal. Get some walks in. When you're confident in your ankle, go for a jog. Personally, I think you should make the 5K a run-walk. Take consistent walk breaks during event. Have fun with it and stay injury-free. Also, I'm just some rando on the internet. If you have serious concerns talk to your doctor.