I strongly agree with the interior patch, Duck Tape, etc. That method has served me very well in other applications. It's not as difficult as it appears to be, just takes patience and thought. Have not used it in patching canvas decoys. Small shot holes no problem with decoy still being used, larger tear, etc. I recover the decoy.
Also was the decoy used for gunning as constructed, or do you want it to be? Just curious.
Canvas working decoys are very durable, practical and toll birds very well. Made to be used, abused, then repaired and recovered when need be. You will find old decoys with shot holes, but few with large patches. Many old decoys have no canvas on them at all, just the frame awaiting the next layer of canvas to get back to work.