It’s always possible someone has saved the original (last) rpm of that app. If not, install ‘rpmrebuild’ from Openrepos (czarkoff) and ‘remake’ the app while you have it installed, then you will have a copy of that app. But as elements of SFOS change, eventually the app may stop working/installing.
Grab rpmrebuild directly with this link;
https://openrepos.net/sites/default/files/packages/4997/rpmrebuild-2.11-2.noarch.rpm
Once installed, open Terminal and simply type ‘rpmrebuild’ and add the name of the app you’re rebuilding.
Example;
rpmrebuild sailfish-weather
Output from terminal;
[nemo@Sailfish ~]$ rpmrebuild sailfish-weather
warning: Could not canonicalize hostname: Sailfish
result: /home/nemo/rpmbuild/RPMS/armv7hl/sailfish-weather-1.0.3-1.8.3.jolla.armv7hl.rpm
[nemo@Sailfish ~]$