Battery Ah selection for a generator
A lead acid battery Ah specification is for when you want the batteries to deliver power for hours. It is not physically capable of outputting that capacity in a few minutes. Assuming the starting procedure takes at most minutes, the capacity is not going to be the limiting factor, you need to look instead at the actual current it can deliver for that time.
Car batteries similarly have at least two specifications, the Ah and the CCA (cold cranking amps). However the latter is for 30 seconds with the voltage above 7.2 V, which I suspect won't be enough for your load.
You need to know more about the generator starter specification. What current it will take, for how long, and what is the permissible drop in voltage over that time. Then get a battery properly specified for that service, 750 kVA is not a trivial piece of kit.
The battery capacity may be relevant if the specification includes repeated starts without the opportunity to charge between them.