The phrase "mostly working" can be replaced by several synonyms depending on the context and tone of the sentence. For instance, if you want to convey a positive connotation, you can use words like "operational," "functioning," or "running smoothly." On the other hand, if you want to express a somewhat negative or less optimistic connotation, you can use phrases like "partially operational," "partly functional," or "working to some extent." Other synonyms that can be used to describe "mostly working" include "partially working," "operating intermittently," or "functioning with limitations." Ultimately, the appropriate synonym will depend on the specific context of the sentence and the tone that you want to convey.