My engines are atlas engines. I assume that is what you mean. I tried looking for some old non working engines on ebay but no luck yet. I know that changes every day.
Atlas sells decorated shells on their website.....but no GP40's, and no NS GP38's.....they do have some SD60 NS shells though. The list changes fairly regularly, so keep checking.....or maybe make a call like UP suggests.
I contacted Atlas for N scale shells. I recently purchased an atlas gp38. They were very helpful through emails since i am making a Norfolk Southern High Hood diesel. The parts were marked for gp38/40/40-2. I also bought a few lil details to add to the engine from them. They have a minimum order of just under $10. My two parts were only like 3.50. Id check with them. I also bought microscale 1987-2002 diesel and shellscale numberboard decals for the diesel. Wish you best of luck.