Because Romney is from MA and because Kerry is one of the leading Democratic experts on foreign policy, there are many things that were easy to find in the DU JK archives. Here are some of them.
Here is an oped that the Manchester Union paper ran that John Kerry wrote recently that very seriously debunks Romney's current positions. http://thepage.time.com/2012/01/08/kerry-reporting-for-duty/ (Interesting in that it is not written like a typical political attack, but a serious attempt to explain the underlying situations.
In 2009, Romney wrote an oped against START that was full of errors in response to a Kerry oped on why it was important. Here (with links to those) is Kerry taking Romney's response apart. The treaty got more than the 67 votes needed. http://www.boston.com/bostonglobe/editorial_opinion/blogs/the_angle/2010/12/new_start_debat.html?comments=all#readerComm
Here is a DU JK thread from back in 2006, when Romney and Kerry argued over Iraq and Afghanistan. It now being 5 years later, it is very clear how wrong Romney was and how correct Kerry was. http://www.democraticunderground.com/discuss/duboard.php?az=show_topic&forum=273&topic_id=98771
What is interesting is that around that time the Boston Herald wrote a very caustic editorial that really went after Romney - as governor and after him lame attempt to gain foreign policy credentials. Oddly
it has been disappeared, but parts of it are here: http://www.democraticunderground.com/discuss/duboard.php?az=show_mesg&forum=273&topic_id=88188&mesg_id=88266