The current version of Kindle for PC is 1.14.0. According to
this page at Amazon versions as old as 1.11.0 are still supported. So it should hopefully it just be a matter of installing a version between 1.11.0 and 1.13.1 (the version prior to 1.14.0). It is a good idea to save the installation file for all software that you are using in case you need to re-install. If you haven't done this then you can still find the installer for version 1.13.1
here. Someone else may be able to help if you require an even older version.
PS: Make sure you turn off the option to automatically update to the latest version or you will be right back where you started.
ETA: In the latest Kindle for PC if you switch to list mode (three lines icon) you will see that the already downloaded books have a clearly visible check mark at the far right. But you can't sort on that.