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Otherwise I think I have to write something using the REST API to do it.
You can use the traditional SFDC SOAP or REST APIs, or set up an SFDC Apex REST service dedicated to this Custom Object.
The way I build such things: Marketo communicates via webhook to a lightweight intermediate gateway (the gateway is necessary because you have to be able to manage identity/access tokens that expire over time). Then the gateway does the actual writes to SFDC (usually while the webhook is still executing, so it can send results back to Marketo).
There's no packaged 3rd-party app to do this that AFAIK (nor would I trust anyone who claims they can set this up in a packaged fashion -- the elements are too delicate and site-specific).