Of course, not replacing my windows means keeping the old ones, which are (again) original to the house, and a bit beaten up. Many of the sashes no longer stay up on their own, and I will have to repair these.
Fortunately, information about that is easier to come by. This video demonstrates how to replace spiral balances in sash windows: https://www.youtube.com/watch?v=Se-YGNegpOo
And Whitco, the Aussie-made spiral balances Bunnings carry, have a product spec and selection guide here: https://www.whitco.com.au/au/en/documents/catalogue-pdf/whitco-window-balance-systems-catalogue/Whitco%20Window%20Balance%20Systems%20Catalogue.pdf