You know, scratch building one of those would be a LOT of fun.
Here's a thought, We've got the Grandt Line Catalog http://www.grandtline.com/
...There could very well be a number of critical parts needed like cylinders, drivers, and other assemblies already available in the On3/30 loco kits. That would be a great place to start. PSC could very well have a bunch of filler items that would be suitable as well.
Using David's Masterclass as a model to emulate for built up assemblies, the prospect of building one could actually be relatively simple.
Years ago I did one of the Grandt Line 0-4-0 Porters. Also in an older Gazzett, somebody did an article on beefing up the frame, in others, built up brass frames. The soldering operation is actually not that bad. The Grandt Line models have wimpy frames (they're cast to scale dimensions).
This could make for a very interesting O Scale MasterClass or Builder's Log! We can easily help with the on-line support that will be needed.