This might not be the easiest way, but it think it will work pretty well for what you are trying to achieve.
In your Marketo Admin area, in the Salesforce integration, "Sync Options", make sure you check "Email opened". What this does is create a Task on the SFDC lead record when they open a Marketo email.
Then in your Salesforce Reports, create an Activity Report, customize the date range and lead owners etc. to your liking, and add a Filter for Subject contains 'opened email'. This will return a SFDC report of any leads who have opened a Marketo email within your specified date range.
You can set this SFDC report to run on a regular schedule and be sent to the sales reps' inboxes if you want.
In short, this enables SFDC to search on Marketo email opens, with the ability to automate the report. If any of the above doesn't make sense, please let me know and I'll try to clarify or post screenshots.