I guess the key word here is TERMINATE. You might be able to get away with stopping and starting your instance again immediately. If it's EBS backed it will be a new instance and will be EXACTLY the same, other than the external IP address. Note that in this case you need to STOP your instance, NOT terminate. Terminate deletes the EBS disk.
To be safe, or if you need to actually terminate, take a snapshot of your EBS volume so you can restore it if you mess things up. It's relatively easy to restore a snapshot to a new VM.
You possibly need to look at why you're being attacked and try to mitigate that before you make your instance available on the internet.