I have been a machinist since 1975.

   Best penetrating oil - PB blaster .   I have used all the military penetrating oil,mouse milk, aerokroil. PB is the best.

  I did jobs on the side when i was in the air force. A co-worker owned a model "T" or "A" and had 13 broke/stuck cylinder head studs. I had previously told him how to do it and he said he worked on it for several weeks. I got out 8 or 9 of the studs be doing the following
     squirt the pb blaster around where the stud bolt goes into parent metal. Get the smallest ball peen hammer you have and LIGHTLY tap the top and keep doing this adding pb blaster as necssary. Clamp a pair of vise grips onto the stud and try gently to turn it BOTH ways.
   If it's broke off flush you have to drill and tap the hole. The pilot hole needs to be centered.