If you enable field history tracking on your lead/contact fields in SFDC then in the history related list you will have a log of what fields changed from old value to new value and by which user. This could potentiall map most of the change data value activities from your Marketo log.
One more question, is that long-text fields won't show full changes?
I've turned on text and long-text fields. Text type fields shows full changes, but long-text field only shows "changed".
For example, Promo Code is text type, Refering Url is long-text type. Lead history only shows following: Changed Promo Code from AAA to BBB. Changed Refering Url.
I reckon it's becasue it might be too long if shows the full history?