Nick, are you doing a repaint or a restoration? In other words, will you need a precise sized decal, or something in the general neighborhood. I do a lot of repainting of 027 types of trains including Alco FA's and have found some of the larger heralds from Microscale HO decal sets to work just fine. In many cases, full O scale decals are too large for the older traditionally sized trains. But you'd probably want to see the actual set of decals to know what would possibly work for you.
I don't think K-Line used a decal on the nose of the Alco FA, but instead used tampo printing. But I'm not positive on that. Somewhere I have an old K-Line dealer parts list, and I don't recall ever seeing a Santa Fe nose decal listed, as you do on Lionel parts listings.
I do have a nose decal for a Lionel Alco FA, but whether it would work or not? The Lionel Santa Fe decal overall measures 3.5 inches wide at its' widest point and 7/8 inch high. The elongated oval with the Santa Fe words in it measures 1-7/8 wide and 5/8 inch high.