Joseph,
I was going to say look into the Microscale line, but it looks like they're getting out of the O-scale decal business as they only have a handful of O-scale decals listed.
I don't know what the UP numberboard numbers looked like so I can't help, but you could probably contact a couple of places to see about getting some custom decals made if none are commercially available.
ELS makes some nice decals:
http://home.mindspring.com/~elstrains/index.html
Also, ProtoCraft, I have some of their Seaboard decals and like them fine:
http://www.protocraft.com/category.cfm?Categoryid=20
Another place is Jerry Glow Decals:
http://home.comcast.net/~jerryglow/decals.html
You may have to send these places some artwork or they may have exactly what you need. I'd email them and see.
You could look for some rub-on type lettering at art type places, take a photo of the numbers (or the engine) and compare to what they have.
I honestly think the days of scratch-building are almost over, not much point in it if you can't get the decals to finish the model.
One more thought, you could try making some yourself. Getting the correct FONT is the biggest hurdle, but most home printers these days give you pretty good results. I've made a couple of decals for some of my Seaboard cars, I have an Epson Workforce 500 printer that makes excellent decals and photos. All you need is the correct FONT, some decal paper, and some Testors Glosscoat to seal the paper after you print. The problem with making your own has always been printing decals that have white in them, not too many printers that are available these days can print white.
This place sells white and clear decal paper:
http://www.beldecal.com/inkjet_decals.cfm
Hope this helps!