Why?
If you are able to fabricate the lower rail and then the lower part of the right hand style, onced epoxied in place you only seem to have a tiny section of stanchion bar to deal with.
Assuming that it is only the tiny bit on the outside that is rotten, you can use the epoxy resin to splice in a tiny bit of timber.
My advice, once all the timber is finished, would be to use a MS polymer (eg
Stixall) rather than putty where needed. You will need to use masking tape on the glass as it is a tad difficult to apply on such as wide/deep section because it is so sticky. You will be able to paint over it after a day or two but you will need to use a waterbased primer first. I often hit ms polymers with two coats of waterbased paint before overcoating with oil based paints. I am not a chemist- but oil based paints directly over ms polymers will stay tacky for months- even if the ms polymer has been in place for over a year.
If the lower internal part of the stanchion needs minor repairs, you can do that with
2 pack filler.