When any boat gets old and needs replacement of mechanical items, wiring, rigging
it can be done. When a wooden boat needs planking replaced or a fiberglass boat needs a deck recore it can also be done. The bigger question is at what point does it become illogical economically.
If the boat is unique, which pretty much rules out production fiberglass boats, it can be worthwhile and budget be damned.
But if the boat isn't unique, regardless of material, will cost more to rebuild than it will ever be worth when finished then it makes less sense.
Remember that a boat that was originally bought in 1970 dollars will cost 2011 dollars to rebuild. Costs can get out of hand quickly in both labor and parts.