Are you able to sync it by changing the name of "Event" within Marketo rather than Salesforce? Marketo already has an object synced called Event, but if you changed it in this new path to "Event 2" or something like that within the custom object sync interface in Marketo, I can't see any reason why it would not sync the object again under this new Marketo custom object. It does mean you have two copies of your Event object synced to Marketo, but it'd get you want you want in terms of targeting?
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