[bojan]

60Hz motor would go slower at 50hz, and that would be 20% slower, the HUGE difference in speed. And if it is AC motor there is no option for changing speed (because motor is connected directly to 240V AC).
I saw quite a few mounts like that in the past.. for example, older Celestron scopes had AC motors with gearbox so the output rotational speed was 1rpm/

Long time ago I helped a friend of mine by making for him a variable frequency oscillator (50-70Hz), and I also had to rewind the motors (to be able to run on 6VAC, from amplifier).
Actually, his mount had two 110VAC motors, connected in series, and both coupled onto the same large gear. I removed one of them, and he was very happy with the results, this scope probably still works as it was modified by me.

Perhaps the best option is to try and find 1rpm AC synchronous motor for 240V AC (ebay?) or to fit in the stepper motor with driver.
I may be able to help here, but I need more information.