I have to make some new battery cables for the D2 24 volt direct diesel start... what diameter battery cable is appropriate to that setup?

Personally I would use 1 or 0 would be better (and appropriate for 12 volt systems).

It’s likely that you could keep the cables short and use 2 gauge cables like a light truck or automobile and since you’re operating at 24 volts it would perform great. 24 volts is high performance! But better to have amp capacity and keep the cables short too. One day the batteries may be low or the weather very cold and the bigger cables might make the difference in getting it started.

I would even go to 2/0. pretty much the standard for equipment/heavy truck etc. Plenty of current capacity with low voltage drop