it is extremely difficult to replace spearheads on copper elements because it isnt usually the spearheads that have 'gone' but the copper elements immediately adjacent and has got thin with pinholes in it. unless you cut back the elements a good way then a repair is impossible and in any event new spearheads need making as almost impossible to retrieve the old ones (how do you recover old spearheads that have been sifbronzed in place?). then you have elements that are too short and according to Jim Ewins' research in the region where the flue temperature is much lower.
so always best to do these things properly and replace completely with new. even better if replaced with stainless radiant elements.
cheers,
julian