"执行停火协定的" can be translated into English as "those responsible for implementing the ceasefire agreement" or "the parties/entities enforcing the ceasefire agreement," depending on the context.
If referring to individuals or groups tasked with carrying out the agreement, "those responsible for implementing the ceasefire agreement" would be appropriate.
If emphasizing the role of enforcing the agreement, "the parties/entities enforcing the ceasefire agreement" could be used.
For a more concise expression in some contexts, you might also say "the implementing parties of the ceasefire agreement."