If the eggs only have a little poop on them you can gently wipe any grime away with a dry cloth or paper towel. Avoid incubating eggs that are badly unless you really have to.
dont 'wash' them or scrub them, either dont use badly soiled eggs or brush away the excess with a tissue gently at most .... ive found cleaning the eggs pretty much in any way greatly reduces the viability .. but dirty eggs have about the same viability as clean eggs, theyre just not pleasant to deal with, so ...
Clean out the nest box and check the condition of the nest periodically. And keep there path to the nest box dry and clean to avoid fresh poop on there feet.