I'll try to conclude:
The driving motor will develop 666.7 kW and the generator will deliver 600 kW.
At pf=0.8 KVA=600/0.8=750; at pf=1 will deliver only 600 kVA.
666.7 kw=666.7/.746=893.7 hp[or bhp] rated.
If the generator deliver only 558 kW the required bhp=558/.9/.746=831, indeed [as already explained].
I think 90% is low for a good manufactured generator [could be 95-98%].But if it is old one, I don't know.