Since Marketo cannot write to accounts, only leads and contacts, syncing activities would have to appear on the lead or contact record. Perhaps there is a way in SFDC of hiding lead and contact-level activities and having them only show on the account.
However, on the account, you can have MSI or activity history display all the activities for the associated account's contacts.
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Also, there is no such thing as a lead account in SFDC Standard. Contacts are linked to accounts, not leads. This means that if you are really talking about leads, then the account has to be a custom field.
The only way to make what you want is to have a trigger in SFDC that, when an activity is created for a contact, it also automatically attached to the related account.