Photo 1;   from the back of the pipes cut  off the outer skin and remove. Use a 1mm slitting disc with a 125mm grinder. The piece shown in the photo's 1 & 2 cut off and discard.

Photo 2;  Use a chisel and hammer to remove most of the cat. Then use a barrel grinder to remove ALL remaining cat material and any sharp edges etc. USE A FACE MASK OF SOME KIND TO PREVENT BREATHING PARTICLES ETC. 

Photo 3;  Re-tig the outer skin back up

PLEASE NOTE:  this is just a general guide/information only.