So, I have an E-Flite Power 32 motor (I'll get to the Cobra). It is possible to take off the collet and set screw and use a arbor press to push the shaft through the can facilitating either front or rear mount. There is a flat spot on both ends of the shaft to accommodate the collet and set screw.

On the Cobra, it uses a little set ring set into a groove in the shaft to hold the can on. But looking at it, it seems if you press the shaft to allow rear mount, there is no groove to put the ring retainer in and so, no way to retain the can. I know you can use the bolt on propeller shaft and rear mount it with the main shaft sticking out the other end, but is there a way to press the main shaft through or is the only option to use the provided bolt on shaft?

Hope that's clear.

you might email Paul,, he has used cobras

I didn't remove the shaft.  I just removed the clip, then pulled the shaft-magnet assembly off (careful not to get your fingers pinched when the magnets pull the parts back together), then pressed the shaft to the new position with an arbor press.  It takes a lot of force.  As Fred said, you have to grind a flat, which is not a big deal.  A collar and set screw come with the motor. I have a bunch of those set screws now.  I'll give you some. 

Don't try using an arbor press to reposition a Plettenberg shaft.  Don't ask.

Thanks, Howard. That should work.