Is your old window difficult or impossible to open or close?  Often it is the result of paint accumulation in between the sash and the parting bead.  Maybe the chord has been torn, making the window slam shut?  While this seems like a major fault, it can be repaired by removal of wooden stops and accessing the weight chamber.
Whatever the case, all these components can be custom milled or replaced.  The benefit is that you can maintain the original infrastructure and upgrade the glass pane.  This can be done at a fraction of the cost of replacement windows.