The two seem quite similar to me, the main difference being the iTablet is just a touch screen and then ModBook has pressure sensitivity.  You could probably find the proper panel for yours if you want the pressure sensitivity, I think that people have been able to use Wacom tablets that they slip behind the lcd screen - this would involve having a functioning lcd, and then the two are separate pieces.  In that case, the best bet would be to get the lcd from ebay or an apple parts dealer and then the tablet panel from here:

http://www.wacom-components.com/english/product/index.html

Or you could try to get in touch with the company that made the panel in the iTablet, TrollTouch.  They make touch enabled iBooks currently:

http://www.trolltouch.com/pages/products/ibook.html

Perhaps you could contact them and see if they would sell you just the panel so you could make a tablet.