A permissive license like MIT/X11 is a perfectly suitable choice. Consider that permissively-licensed code may be freely combined with proprietary components. The situation is no different here. (If you wanted to use a copyleft license like the GNU GPL, you might need to grant an exception to allow distribution of your GPL'd code with the nonfree assets, but a permissive license like MIT/X11 already grants such permission by default.)
Where you need to take care is in clearly indicating which files are licensed which way, and communicating to recipients that the project as-is cannot be redistributed for commercial purposes due to the proprietary license on the art assets. Communicating this information is not within the purview of a license document itself, which indicates what use is allowed, but belongs in a separate document that says which license terms correspond to which files. Include this information in your
LICENSING file, or other prominent document.