Would people be close to the bell? My first thought was of a picture of a bell from the Haunted mansion that was being "rung be ghosts". It looked like a bell but just had a speaker inside that played sounds of a ringing bell.
How big is the bell?
Could a small motor fit inside of it, with a cam on the axle?
If not, could a motor fit behind it where it couldn't be seen, again with a cam on the axle that hits the bell when the motor spins?
If you want a heavt-duty bell-ringer find a power tool like a Wagner paint scraper and mount it solidly next to the bell,mount the bell solidly too. but ,of course it still has to"swing' some.
The paint scraper has a small blade that moved back and forth very quickly with strength. The bell will get "Tired" before the paint scraper does.
I bought mine at Goodwill ,brand new in the box for $2.00? 120volt tool.