I seem to have acquired 2 body parts for a Bearclaw-c in pocket planes, and even bought a Bearclaw-c plane but on the parts Menu and on the build section I can't see how to combine the 2
You have to have the three separate parts for a plane type in order to assemble it:
The plane types must match exactly, so e.g. a Bearclaw-C engine won't combine with a Bearclaw-P body.
When you have all three parts for a plane, the "Build" button at the bottom of the screen will show you a listing of planes for which you have parts, but the one(s) for which you have a complete set you can convert into an aircraft for the right number of bux, which varies based on the plane.