Can someone help with a moped issue?

I have 1977 motobecane model 50v. The magneto bolt is supposed to be a square head and left hand thread. Well someone stripped out the square head and then cut it so now its long and flat, and i don't know how i would get the magneto bolt off now because i want to purchase a new one. I tried taking a chisel and using a hammer to bang it out its really in there