Author's Note: This was great fun to do. Thanks to RoseyShades for coming up with the idea and contest.
I managed to order a high resolution digital copy of the drawing I wanted from the British Museum, and recieved email permission from them to alter and colourise it for this contest (as it went beyond their standard terms and conditions).
Here's a link to the original drawing, and here's a WIP.
Original image © Trustees of the British Museum. Used with permission.