I built a HO scale 209 out of an Atlas RS3 many years before, using an article on how to model Southern RS3s. Sorry, I dont recall which magazone that was, but it was in the later 1980s.
Anyway, I used a microscale Southern RR diesel decal set, the one that had the ampersand in Savanna and Atlanta. I built the "W" by chopping up two letter As from that set.
I sold it off when I left HO for good in the 90s.
Masking off the body for the orange stripe was easy. You could also use a pre-marked SRR one and use chemicals to remove the letters and numbers, to represent either 209 or 210 before the orange stripe was added to both.