Apologies for the double posting, but here's a link to a collage of reference images of the model (taken by John Eaves during its Lakota
state) along with the newly discovered part.
Clearly there were some modifications, but indeed it most certainly looks to be it. They added some embellishments, and it looks like they built the box that receives the mounting arm onto the flat, wider part.
Perhaps most interestingly, left off the cylinder part that screamed engine.
Whew. That was close.