Just pound out the remaining part, then march down to the local auto parts store for a new one. You can even pound some out at the local salvage yard if they have the same model car. Install a nut to bang on so you don't wreck the threads. To draw the new ones in, notice there are serrations under the head to bite into the hub. Install a large nut or a stack of large washers with center holes bigger than those serrations so they don't get caught. You'll need a nut without a chrome center cap. Install it backwards with the flat side against that stack of washers, then tighten it to pull the stud in. If there isn't room behind the hub to get the stud in and out, look for a part of the metal plate behind it with grooves showing where to knock out the piece to create an opening. You might not get the new stud all the way in. Install the wheel, then use a click-type torque wrench to tighten the nuts. Typical values are around 95 foot pounds for standard studs and 80 foot pounds for anodized studs. Anodized studs are used a lot on imports. They will be silver, light blue, or light yellow. That is an electrical coating. There should be no lubricants on them because that coating is a lubricant already. The use of the torque wrench is extremely important. Many shop owners will warn an employee once for failing to use one, and they will fire them after the second offense; it's that important. At roughly 105 to 120 foot pounds, (which might not feel tight if you're using a long breaker bar), the nuts will just keep turning but you won't feel that the threads are being peeled off the studs and nuts. You'll find that out the next time you try to remove them. Besides holding the wheel tight and not damaging the threads, proper torque is important so a 90 pound person can get them loose to change a flat tire, and so the clamping forces are equal all the way around to prevent warping the rotors. If you have standard steel studs it is acceptable to put a LIGHT coating of grease on them to prevent corrosion but here are some real important precautions. NEVER, never, ever use anti-seize compound on lug nuts and studs, even if your boss tells you to. There is a chance nothing bad will happen, . . . on this car, but you don't want to risk your reputation and time spent in a courtroom when that results in a crash. Along with that, if you DO use any type of axle grease on the studs, you must turn the nuts on by hand as far as possible. Do not use air tools. The grease will build up in front of the nuts as they thread on. If you use an air impact wrench, the centrifugal force will spin that grease onto the rounded surface that contacts the wheel. That is the friction surface that holds the nut tight. Grease on that friction surface can help the nut work loose. Anti-seize compound on the friction surface WILL cause the nut to come loose. During our day of orientation, I spent a good 15 minutes telling my students about anti-seize compound and torquing lug nuts. Given that they were warned, if I caught anyone not following proper procedures, they were not allowed to move the car until that compound was washed off, and it had an impact on their final grade. I'm the most easy-going instructor you'll ever find, except when it comes to safety and the chance of being responsible for a lawsuit for your boss.
In case the new studs might not have pulled all the way in, retorque the nuts after you drive the car for a mile or two, and again after around 20 to 50 miles. If you find any nuts that turn a little after you retorque them, check them again in another, oh, . . . 10 to 20 miles until they are always tight.