Ok I can help on this one!
Find a nut thats the right size and put it on the stud 1or2 threads and grab the mag and weld the middle the stud first! Then go to the nut should sound like eggs being fryed. I have fixed many things with this trick works well for me and a oldtimer showed me. Then turn the nut/stuf out should come right out, then order the right stud from the moped dealer of you choosing. I don't think this will hurt the cylinder or piston but ask the pro's!