Rats. It's A bit newer than I wanted. Maybe I should sell it.
What's your era? I'd say the only things that require change for an earlier era would be to remove the shield on the side of the cab and replace that with an appropriate slogan (depends on the year)...and maybe paint the trucks Harbor Mist Grey if before March 29, 1955.
The first GP9's to arrive had the slogan "Road of the Streamliners" on the engineer's side of the cab, and "Serves All The West" on the fireman's side of the cab. In June of 1959 the cab slogan was changed to "Dependable Transportation" on both sides of the cab. In late December 1972 GP-9's were the first engines to get the new slogan "We Can Handle It" on their cabs.
Interestingly, all diesels that got new paint and new slogans starting in late December 1972 had no black edging on either the slogans, numbers or other lettering in order to save money. However, management took a look at the new edgeless red lettering and a new order to restore the black edging on all red lettering was issued in March of 1973...basically three months after the edgeless change had been started. Of equal interest is that some of the GP9's never got the newest lettering with the black edges and retained the plain red lettering until they were retired...even after the cab slogan was replaced by the UP Medallion.
In October of 1975, all cab slogans (with a few exceptions on engines other than GP7's and GP9's) were changed to a 24" UP Medallion...like is on the the OP's Atlas GP9. GP7's and GP9's retained their original size "Union Pacific" and numbering because of space limitations.
In April thru June of 1978, all road diesels were required to have white vertical handrails and step edges, replacing the ones that had been painted Harbor Mist Grey. Switch units had been ordered to have their vertical handrails and step edges painted white two years earlier starting in April 1976.
Other modifications to make this model more accurate would be to add wipers, a cab vent, mirrors, cab window shades (canvas), amber strobe, armrest, repositioning the horns according to reference photos, extended exhaust stacks, cold weather cover, separate grabs, lift rings and MU stands...all depending on the era and according to reference photos as details varied amongst individual engines.
One last thing to consider. UP's GP9's in this number series had the same handrail stanchions as the earlier GP7's...and it's an easy replacement to do if you can find Atlas GP7 handrails. I do it to all of my UP GP9's.
Here's an early Atlas GP9 that I superdetailed and got it right on for sometime in late 1955 thru 1956...
Photo (1) - Early Atlas GP9 superdetailed a bit with some of the aforementioned additions and paint/lettering modifications:
Cheerio!
Bob Gilmore
