specific confirmation email for an event?
Two options!

Use the text in the Event Config -> Online Registration -> Confirmation Email -> this is Event specific and this text/note will be pasted ahead of the receipt Email - so ahead of what is generated by the message template.

Or - if you want to go into more detail:

You can get into the message-template and add some if logic - that would essentially change the message based on {$event.event_xxx}