Hey
Hirohawa ! I don't have one of these so take this thought with a grain of salt.
From pictures on-line, it kind of looks like there are two pins towards the bottom of the clip down near the buttplate. Have you tried trying to punch them out yet? Be mindful that they may not be pins, in the sense of say the ones found in an AR-15, but rather nubs on another part protruding through the wall of the clip sides, and that using a punch on them might not be the best idea. Especially if they're attached to the buttplate and that's what holds it within the bottom of the clip sides.
Regardless, assuming you can get the buttplate off somehow without damaging it, I would then cut off material from the bottom of the clip in order to reuse the top section so that the clip could still be used with whatever locks it in place when the top of clip is inserted into the pistol.
You will probably have to address the issue of the little semi-sphere bubble that stops the clip during its insertion into the pistol too. If the clip is plastic you could just grind the little half moon shape right off of there! If it's metal, and you're really crafty, you could insert a hard wooden, metal, or plastic plug, and gently tap the protrusion flat. You might need to slit it in the center and remove a thin "pizza slice" out of it first so that the metal doesn't put outward pressure on the body of the clip on that side and make it too wide to fit back within the pistol. You could solder or bondo the crease/seam after it's flat.
Drill some new holes to reuse the buttplate "pins" (if that's how it's held in place), then sand and paint it to make it look pretty again.
Once you reattach the buttplate, it will act as the stop, the same way that the little half moon shape did.
Good Luck! I hope this helps or gives you ideas on how to achieve your goal.