Hi.
Carefully drill out the stub of the broken off spring tube, and replace with a suitable piece of brass tubing. This can be soldered or screwed into place. Find or make a piece of brass rod to be an easy fit into the tube, and round off its end, and then cut it to about 8mm long. All you then need is a suitable compression spring that will fit into the new tube. Obviously the tube needs to be long enough to retain the plunger when the adjuster Knob is in its usual position, and not too long or it will foul the serrations on the underside of the knob. I can’t give you the correct length as with years of wear the best length may now be less than the original. This repair can also be applied to chain oil drippers as the bits are very similar.
