Yes you can get into it. it is a pain in the ass, not an easy task, but possible. You do NOT have to cut anything. I just did this to insall an SPA tach.
take off the chrome cup and you will see how the aluminum bezel is wrapped/folded around the white plastic cup that encases the speedo. i used a small screw driver and a very small prybar to unfold the bezel. when i closed it back up i used a couple different diameter punches to push the fold back into place.
i dont know if you can fix the tripmeter knob.
pesonally i would just leave it, too much risk in damaging you speedo or the bezel, just to have a working trip knob.
the bezel may not fold back into place nicely and you will have exposed creases when done, ive seen it on my friends bike