As Ed said, replacing the whole shebang would be a daunting undertaking. On the plus side, I am intending to rebed most of my hardware (over time) anyway, to ensure the core stays dry (drill oversize hole, fill with epoxy, redrill - so no exposed plywood)
My rubrails are 1/8"+ thinner than they started out, but still usable (barely). I now don't sand them too much before finishing, as there is so little wood left, so the ridges show under close inspection - but it looks great a few feet away. However, I did have to repair a 2"x 4" chunk out of one side....I used a "dutchman" repair (like this:
http://www.blacksashvt.com/portfolio/charles-keeler-house-window-restoration/4242618). So depending on the pieces needing replacement, some repairs may be possible. A competent woodworker can do a lot, but teak is EXPENSIVE (at least in CA), and you need to look at the remaining wood to see if it is worth doing the repair.
I wonder how the top and bottom/side pieces are joined? Replacement of a side piece *could* be fairly straightforward; a strip of teak cut to size and shape, bent into place - the curve is very gentle - with scarf joints feathering the repair to the remaining wood. Much easier than the top rail due to the orientation of the grain. Screws and a gentle adhesive/caulking (4200 maybe?) would hold it.
I bet they steamed big pieces for the top rail, clamped them until they cooled, and then ripped them to make boards for the curves. Economies of scale would have made it relatively inexpensive.