This has probably been an issue for a long time, unnoticed. It only effects the widget version of the top surnames block, so perhaps not entirely related to the change in the php version.
When I created it, based on a copy of the Home page top surnames block, I left three lines of code still using a function from the original block in place. That’s been fine for me because I have both blocks active. But I see now that if the Home page version is not active, then the widget version will fail as per your report. Sorry about that.
The fix is easy:
Open the file kiwitrees/modules_v4/widget_surnames/module.php.
At lines 99, 103, and 108 is the phrase “top10_surnames_KT_Module“
Change this to “widget_surnames_KT_Module” on all those three lines and save the file back to your server.