Push command not working says permission denied
If you have a backup font file and adb is working, then you can just do adb remount and try pushing the backup font again. This command is just a shortcut for adb shell mount -o remount.rw /system, which remounts /system as writable.

If adb remount doesn't work, then run adb root and try again. (i.e., run adb root, adb remount, and adb push /path/to/backup/Roboto-Regular.ttf /system/fonts/, in that order). I'm not sure how picky adb is about telling it to use root.