As quoted from the Football Champions game page itself:
"If you want to continue playing [after Dec. 31st], please download the game on android or ios (you can retreive your account by opening the option menu on the mobile version, then click on Facebook). If you really want to play on windows, you can download the game by clicking here
(note that you have to unzip it after download)."
I replaced the link they gave with an Anonfile upload, you can check on the website that it is the same exact .zip file. It's pretty much just a download of chromium with the flash plugin, so after the 31st it will still be intact.