Morning Ironbark, I have a Ford harmonic balancer puller as well. I have used it with 1/4 bolts on other flywheels.
I've heated and let WD40 run down crack..... a few times...... I'm not in a hurry.
There's not much data around about this old Phelon magneto.... but I bet it's the condenser.........
speedy

........................Keep your blades sharp......................