Hum... I think it depends of the game. If the game jumps to 60 Hz and there is no option to change then you will not been able to play at 50 Hz. Maybe... is there a TSR (resident) program that enables this option using a mapped key?
I think the game was intended to play on 60 hz... if you will play it on 50hz the gameplay will not be smooth and the music runs to slow...
but why 50hz??? Do you use an old colour television that goes LSD/Mushroom when it hits 60hz??? I had one in the early days.... getting pretty mad and bought a philip VGxxxx rgb monitor.. problem solved...