From a 'looks' point of view, YES! keep them. from a replacement angle, there would be quite some 'making good' to the interior surface's, also, a point to bear in mind, if? they are large, i.e. 5 or 6ft wide and on the ground floor, some served in a slight structural way, they provided some suport to the internal timber lintol, which a uPVC replacement will not be able to do.
There are a few comps out there that specialise in the total referbishment of shash windows, they can draft proof them, some might be able to install 'stepped' double glazed units.

Just in case you are still unsure, there are people out there that are starting to replace uPVC windows with ...... timber original pattern ones!
The worm is starting to turn.
