Not sure what sling they used on a Krag carbine, but the rifle one was like the Trapdoor. A 1903 sling will fit, but it's not the same, the Krag sling is one long strip of leather, not in 2 parts like the '03. The sling I have is from S&S firearms, it's OK but not great. it's made from 2 pieces of leather, spliced together, and it's color is a little "orangeish". I think the ultimate one comes from Turner Saddlery, but they may not always have them in stock, takes a good size hide to make one strap that long.
The Spanish used a rectangular black cartridge box (or a pair of them) for both the Roller and Mauser. But for Mexican revolutionary, anything goes, and 2 crossed bandoleers and a big sombrero would work fine.