Well for starters, most of those illustrations above won't work as is. True O scale decals will be far too large for a 027 Alco, especially those with nose decals like the Erie Lackawanna... I know for certain... I held a set up to an 027 Alco shell. Or the NYC scheme as another example that I've tried out: The HO ones are a tad small, but workable. The O scale ones are way too big.
First off, what road names do you like? Secondly, you're going to have to take some liberties and approximate or replicate the paint scheme you like. You're going to probably end up using larger herald HO decals (like from an HO locomotive or hi-cube box car set). Or if you can even find O scale or S scale decals in the road you want, you'll have to be selective about what you use: It's unlikely you'd use all the decals in one of those sets.
Here's a spattering of some of my own repainted Alco FA's. These here are all K-Line. And just remember as you look at these, O scale decals would be double the size, and in most cases, wouldn't work at all. But let these inspire your own creativity. Then you'll have to hunt around and see if you can find decals for what you wish to do.
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First off below, a Conrail unit painted with Krylon Shortcuts Ocean Blue, with a paint mask for the second black color along the bottom. The decals are Microscale HO ones.
Next below was inspired by the actual prototype NS 999. But since NS has no Alco FA units, and since an O scale decal set would be too large (if this particular scheme is EVEN available in O) and the HO set too small, I took some liberties and did a simplified "approximation" of the actual 999, numbering mine "9990" since the Norfolk Southern has zero Alco FA's. I do have to do the number board still for this one.
This CSX MOW paint scheme is one I actually made the decals for on a Xerox type copy machine.
I'm a huge fan of the Lehigh Valley. The LV never did this paint scheme on an Alco FA, so again, liberties taken. Decals are a mix from various Microscale HO sets.
This next one is one of the very first engine repaints I ever did. Inspired by a question, what if the Penn Central hadn't been so cash-strapped, might they have done some different paint schemes other than straight black. Once again, Microscale HO decals.
More paint masking done on this one. I was really happy that there was zero bleed on this particular one. I do wish the grey had been a little darker in light of how poorly the particular yellow of the decals shows up. Had I had extra exact decals, I probably would have doubled them up with another exact one placed on top of the other. I've done that in the past a couple times and it worked great to get them darker. And once again, liberties taken as the LV Yellowjacket scheme was never applied to an Alco FA.
Here's an easy one to do if you want a single black color. Just make sure the paint has a gloss finish for the decals to adhere properly. I should note that one this one and the Enviro 999 scheme above, I used an S scale decal for the nose. Someone at one time was making NS engine decal sets in S (I think Shellscale), so I bought a bunch of them.
I've also used the HO Norfolk Southern nose decals, and sliced them to extend the 5 lines a little further.
