Well there is only one message template for event confirmation following a online registration. So if you change it, it will apply on every event unless you add some smarty code in it.

Another method is to disable the confirmation email for this event and use schedule reminders instead. 

 * administer -> communication -> schedule reminder
 * entity => Activity - Event registration - Completed
 * create a schedule reminder that start 0 hour after Activity Date Time
 * don't forget to enable cron job : Administer -> System settings -> Schedule Jobs and set `Send Scheduled Reminders` to `every time cron job run`

As it is an automatic process, i recommend to do some tests before activate it widely.

**EDIT** This will not work because you can't specify which event will trigger the reminder.