I have shortened several 1907 scabbards to fit scabbard-less short Indian smle bayonets and I have found that flat bladed screw drivers will turn over the staples quite well. I mark the shank of the screw driver with a black marker pen with the widest part of the blade lined up with the staples on the outside of the scabbard. I then insert the screwdriver into the scabbard throat up to the mark on the shank and twist the screw driver 90 degrees. You need to pick a screw driver that has a shank that will fit through the throat and a screwdriver blade wide enough to turn the staples over and you also need a bit of judgement on how to line it up but it seems to work quite well.
If you wish to make a mandrel to fit down the full length of the 1907 scabbard, this is a fairly simple task because the metal required is a standard stock size. I made one from 6mm x 25mm black mild steel which has rounded edges and all that is required is to shape the end a little and round it off. This will do the staples on the chape, if required.