During the initial wave of "baby carrier" hate, I saw a few types of cockpit enclosure conversions emerge. The first involved slapping a cut down
IG Sentinel cab onto the chest, turning it into a more humanoid mini-Warhound. The second method was to turn it into a giant Dreadnought, using either Dread or Venenerable Dread parts (depending on whether you wanted to see a pilot's head) in place of the dangling pilot. You can see an example of the first style
here and
this thread on B&C has examples of the second.
Unfortunately, neither looks like it'll get you particularly close to a Titanfall mech. Breaking out the styrene and putty might be your best option. As evidenced by Dakka's own heavybolter in
this thread, you can pretty drastically change the feel of the kit by reworking the chest area (obviously, he went a bit further than just that

).