I found a fix for this issue.
Rebooting the phone and going back into notes to define a word allowed me to access the dictionary picker. From there I was able to get the Apple dictionary and the OED. Subsequent definitions worked without freezing the app.
From then on, I was able to switch to the system dictionary in Marvin and look up words without any issues. However, I noticed the Marvin dictionary popup uses default iOS 6 styling rather than iOS 7's new look. See below for comparison.
Marvin:
http://i.imgur.com/zC66K4A.jpg
Notes:
http://i.imgur.com/Q2ficP2.jpg