Whereabouts in the country are you.
If you wanted to consider making replacement sashes, I might be able to help machining the components up for you to assemble etc.....it might depend how busy I am
It might work out better for you than all the time spent in remachining the existing.
If you want go reaching the existing, have a go at a small window first- post some pics on here and I would be happy to advise (and others of course). I used to own a Joinery company and currently do work on period properties so it's kind of my thing