The Kaup site is pretty good but in metrics. many US models can not be found either (unless they fixed this problem since I last saw it)

One simple quick way I use is to convert the increase in load center to a %. Going from 24" to 30" means that you increase the LC by 25%. Now deduct 25% from the original capacity of the truck. For example a 5000lb truck will now be 3750lb. Don't forget about the downrating of the truck. at 200" a 5000lb truck might only have a residual capacity of 4500lb which at 30" lc will be reduced to 3375lb.

This is a very unscientific way that will get you in the ballgame. I would always consult the manufacturer before quoting your customer.

A scary thing is that forklift trucks are unable to determine load centers. A 5000lb truck will lift this weight no matter what the LC is. At least until the truck tips over. Make sure the capacity of the truck is limited to accomodate the changed LC.