Keeping face warm in the winter
A trick that I found is used by NFL players to keep their face warm :
"A jar of Vaseline is an old trick from veteran players to block out the wind"
source: https://www.espn.com/nfl/story/_/id/14525394/seven-ways-nfl-players-stay-warm-frigid-conditions-nfl


BONUS: If you get too hot with the vaseline on you can use the excess to lubricate your drivetrain.

I think some skiing/snowboard goggles with a clear lense will also help in keeping your face warm.
Something like this:
https://mx-deals.nl/oakley-crossbril-o-frame-jet-black-clear-lens.html?gclid=CjwKCAiAluLvBRASEiwAAbX3GU82TCnNSiATDbKyg5txyfgQ22RLn0RFJsY-fwpcHCIu53Mn8MTdwxoCEVkQAvD_BwE



A balaclava looks like it would keep you warm,

you might draw unwanted attention though.



This might be a more subtle but similar alternative: 


There are even versions of balaclavas which have heating elements inside:

                
            
https://www.thewarmingstore.com/heated-balaclava.html


A full face helmet will definitely also help in keeping you warm, but it's not the most practical solution. I've done a quick search regarding heated motorcycle helmets and could not find much so I assume it will not be needed even in cold conditions.
One comment I found on a forum thread regarding heated motorcycle helmets (to illustrate their effectiveness at keeping your head warm): "Are you serious? Why would you actually need that? A helmet acts like a thermos for your head because of the EPS liner. Keeps you warm in the winter and cool in the summer."
source: https://advrider.com/f/threads/heated-cooled-helmet.639790/

There are heated face shields for full face helmets but I believe their main purpose is to prevent frosting/condensation on the face shield itself, not so much to keep your head warm. They look like this: