Hi I managed to get apt working, the first thing to do is edit your sources list to use the debian archive repos
sudo nano /etc/apt/sources.list
First we want just the debian repos no osmc repo, the signing keys are expired so we need to set them as trusted
deb [trusted=yes] http://archive.debian.org/debian/ jessie main contrib non-free
deb [trusted=yes] http://archive.debian.org/debian/ jessie-backports main contrib non-free
deb [trusted=yes] http://archive.debian.org/debian-security/ jessie/updates main contrib non-free
sudo apt update
sudo apt install apt-transport-https
After updating you can add the osmc repo back
sudo nano /etc/apt/sources.list
deb [trusted=yes] http://archive.debian.org/debian/ jessie main contrib non-free
deb [trusted=yes] http://archive.debian.org/debian/ jessie-backports main contrib non-free
deb [trusted=yes] http://archive.debian.org/debian-security/ jessie/updates main contrib non-free
deb [trusted=yes] http://apt.osmc.tv jessie main
Do not run sudo apt-get dist-upgrade i broke my install doing this and had to reinstall, it bootloops with a sadface so i think it fails to open kodi