All engine powered generator usually rated in kVA
from your spec given on picture, the maximmum is 65kVA, and minimum 24kVA.
load calculation is actually can be calculated by Ohm law.
but there is common procedure the maximum rated capacity only can be achieved by increasing the engine's speed / rpm to the max and for short period of time (<30minutes of full load).
which is not even worth it to try, either your engine will be blown or the generator will overheat.
thumb rules of basic generator : for continous operation only 50% from Full Cap Load. which is on your generator is 32.5kVA
and remember kVA is apparent power generated by generator. say, if your pump (load) is rated at 20HP = 14914Watt is not loaded on your generator by 14.9kVA but more than that.
which is calculated by your pump power factor. for your consideration the old pump usually have 0.5~0.7PF respectively with deaging starting capacitor on your pump + loading in your pump (water inside the turbine).
so 14,9/0.5(put the worst number of your PF just in case) = 29.8kVA (it's almost reaching the margin of your reccomended load). but that will be reduce after the motor fully spinning (PF increasing linear with the load in your pump decreased).
If you have the good oldie pump with 0.9PF don't worry, your generator is big enough for 30hp+20hp = 37285W = 41.43kVA = 63.74% from Full load.