Can an emails stats (number delivered; number open; number click) be written from Marketo into a SF custom object?
Previous marketing automation tools I've used we could do this so that some reporting could be pulled from SF and a sales team could see the impact an email had.
Via webhook, yes.
Actually building exactly this for a client, where certain totals get rolled up to the Campaign object. The webhook part that sends the events from Marketo is very easy, though. The complexity is that you have to create an SFDC endpoint (webhook-compatible) that can process those requests and bump the fields accordingly.