Suitable Substitute for this Varistor MDC Z750 / 40
You can use any varistor with a working voltage a few volts higher than the voltage of the line where it's connected. Check the voltage of the line or wire. If it's 36V, then search for a 38 to 45 working voltage varistor.
Attention: Not the clamping voltage which is far higher. The varistor starts to react gradually above the working voltage. Then clamps somewhere around 50% above the working voltage. Don't mix the two numbers. The working voltage is maximum at which you can use. A clamping voltage of 40V will be way too low for working with 36V. So I guess, 40V is the working voltage  of this varistor.
Modern varistors will be several times stronger and better than the original one.