This may or may not work, but if you have photoshop you could try to resize the image as a 600 pixel resolution, then see if it will look a little better enlarged. If you don't have PS, you can send me the file and I'll try. Can't hurt.
...I tried it by saving the image posted above and it wasn't enough of an improvement to really make a difference. I'll still play around with it and see if I can figure anything out. Maybe you'll find somebody in the meantime who has an original.