Wish I could say that you could use this or that but the fact is this, GM&O is apparently not as common of a roadname that is desired than many others. I have that this is looking to be the way it is but there is little possible... except maybe...
Have you considered using dry transfer lettering?
Clover House offers some great alphabet and number sets that would allow you to create decals or apply the lettering, one letter at a time, directly onto a model. While it is time consuming and a little tedious it isn't really all that hard to do and since the GM&O's steam was rather simplistic in nature it wouldn't be all that bad of an alternative.
Finding photos on the web isn't all that easy but George Elwood's Fallen Flags site does have a few shots of GM&O steam and the lettering is about as minimal as any I have ever seen on a streamer. Here are a couple of links...
GM&O 475 2-8-2GM&O 502 4-8-2GM&O 547 2-8-0Two exceptions I could find about GM&O steam won't be nearly as easy to do... the first (below) being steamers that had the slogan "The Alton Route" while the second is a maroon painted passenger steamer based on the Alton RR's steam paint scheme...
GM&O 2998 2-8-0GM&O 4-6-2 "The Alton Route" (model)
Don Ross has a number of additional shots on his great site...
http://donsdepot.donrossgroup.net/dr2083.htm
