I happen to have my MPC open the carburetor section so I looked. The carb in question, C7DF-V, uses parts list Sect95.11V. The top cover of the carburetor use a different part number, C7AX-9524-A, which is only common to another carburetor documented in section 95.11R. A quick cross reference show a full size Ford, 289 2V AT with smog application. In other words, not many applications to chose from. That's the why, I don't known the where or how to get a replacement plug.
Jim