I would not use glue or epoxy. My guns are as historically accurate as I can reasonably make them.
I have soldered a thin, hardened piece of handsaw blade to a frizzen and it works fine. The trick is to anneal the frizzen and file it back at least the thickness of the steel you will add to the face. Otherwise the frizze may hit the lock’s flashguard or you change the geometry.
Then fit a piece of thin annealed high carbon steel (0.015 to 0.030” thick) carefully to match the frizzen face curve. Then trim the “half sole” close to fit. Then harden the frizzen and the sole (yes you need the frizzen toe where it engages the frizzen spring to be hard). MAKE SURE THE NEW FACE IS HARD AS GLASS.
Now flux the frizzen and with a large soldering iron sweat a thin coat of low temp solder on the frizzen face (tin it). Wipe off excess with a rag or brush.
Flux the new frizzen face in place and wire it in place. Hold the toe of the frizzen in a vise with the frizzen face more or less horizontal and heat slowly from below with a torch. When the solder just melts and the frizzen face sits down, slap it with a wet rag. Mind the steam.
Temper the frizzen toe to just blue. You should be good to go.