Not cilantro or ginger. The cilantro and ginger are combined together with a mortar and pestle. The flavors come together and act as a substitute. The OP asked for substitutes. Not exact replicas.yes, like pat says.
I also don't know the ratio. Bit difficult anyway as not every lemon grass stalk is the same size and same intensity.
If you would have neither, you can use some extra lemon on lime juice or zest.
Or leave it out.
sgsvirgil : substituting lemon grass with cilantro or ginger?
That would mean changing a recipe quite a bit. Those flavours are totally different!