It is reported that two batteries are to be used in series for 24 volts, as is common practice for starter batteries for large engines.

There is no indication that the original cables have failed, new cables are reported to be needed as the batteries are to be relocated further away 

I therefore stand by my original suggestion that the new cables be AT LEAST one size larger, and that two sizes larger might be better.

It is not common practice to provide any OCPD for engine starter circuits, and great care is therefore needed to reduce the risk of a short circuit to the absolute minimum. My usual approach is to place the positive and negative battery cables each in their own insulated conduit, pliable or rigid plastic as seems best.

A few meters of such large single cable, and enclosure in insulating conduit is a trivial expense compared to the costs of failure to start the engine reliably when called upon. placing the batteries further away from the engine reduces failures resulting from heat and vibration, and eases future battery replacement in the event that new batteries are larger or of a different shape.

It is reported that two batteries are to be used in series for 24 volts, as is common practice for starter batteries for large engines.

There is no indication that the original cables have failed, new cables are reported to be needed as the batteries are to be relocated further away 

I therefore stand by my original suggestion that the new cables be AT LEAST one size larger, and that two sizes larger might be better.

It is not common practice to provide any OCPD for engine starter circuits, and great care is therefore needed to reduce the risk of a short circuit to the absolute minimum. My usual approach is to place the positive and negative battery cables each in their own insulated conduit, pliable or rigid plastic as seems best.

A few meters of such large single cable, and enclosure in insulating conduit is a trivial expense compared to the costs of failure to start the engine reliably when called upon. placing the batteries further away from the engine reduces failures resulting from heat and vibration, and eases future battery replacement in the event that new batteries are larger or of a different shape.

Agree - prompt starting is everything with a genset. Looking at likely engine sizes, I'd expect something about the size of the Cummins KTA50 GS8

https://www.cummins.com/sites/default/files/2019-08/KTA50-GS8.pdf though it may well be another engine maker's offering it will be a  similar horsepower, so about a 50 liter machine. Looking then at the installers spec

(excerpt below)

we see that we are allowed no more than 0.002 ohms in the cranking loop. (To put that into perspective, that's a couple of volts drop at 1000 amps, 1 volt at 500A so not so insane)

Now we do not know the engine on this genset is the same, nor that the starter is the same, however  there are not many makers, I don't have a data sheet but the Perkins model P1650 GW would be similar,  (and I don't think Kubota make anything large enough) and the figures allow us to start to estimate.

An 8m loop of cable there and back will be 16-18milliohms * 8 ~  0.15 ohms  per mm2 of cross-section, so 10mm2 is clearly too small at  0.015 ohms, (but we knew that, we are not starting a Mini !! ) and you are up to around 100mm2 before it feels sensible at 0.0015  ohms.

So 95 to 120mm2 or bigger rubber covered welding style cable seems about the right sort of thing, to me at least, to be comfortable.

Comfortable is relative - do you have a good hydraulic crimper to put lugs on ? Termination resistances will be critical.

And, as per Broadgage, I'd be putting at least the long run parts of each wire  into some sort of tough plastic tube or flexi duct, and anchoring things well away from any possible chafing points.

A fire at the sort of currents that this could deliver does not really bear thinking about.

Thermal de-rating of the cable is not so relevant, as the starting duration is so short.

Mike