Get some nuts of the correct size... take the valve cover off to access the threads closer to the head and hopefully not stripped...

If you own a stud remover, use it with your ratchet to take out the stud and go to a good bolt house and see if they can match it up...

If you do not have a stud remover, then take two good nuts and run them all the way down on the stud... then use two wrenches to tighten them together,, then use the lower one to undo the stud.... by trying to take it off the stud while the top nut is in place...so you are using the friction between the two nuts to hold onto the stud's threads to take it out.

If that does not work then try using a file on opposite side of the threads.... to where you can either use a wrench or visegrips to take out the stud...

Then put a good stud in.. using red locktite on the bottom threads going into the head.

leave overnight.

then install the valvecover and nuts on the new studs....