Run script as root without sudo

I've got some scripts which are intended to run on a new EC2 instance 
right after it is created. Since the script needs to install packages it 
need to run as root. But because I don't have sudo installed at this 
point (it is a brand new instance), I've only got 'su' to get root.

The script itself is launched over SSH with the ec2-user account and 
there is no root password at this point in the startup.

My first attempt was to put this inside the script itself:

if ["$($whoami)" !="root" ];thenexec su -c"$0" exit1 fi


But su complains that I'm not allowed to execute a command using the -c 
option as root.

How else can I get this script running as root remotely in a completely 
unattended way?



Cheers

Ari Maniatis