Loads determine PF, yes.
All calcs are carried out assuming certain parameters such as voltages stay the same, current to the load varies in amplitude and phase shift/power factor. We also assume that when calculating the pf, that the source is at unity.

Whilst in reality all these factor vary slightly, our calcs tend not to show this, but for the sake of my question I am ignoring such fluctuations. My question I pose today, is that if I have a load, say a motor, an escalator perhaps, that is rated at say 50kW, it shall run at 62.5kVA and draw say 90Amps when connected to the mains.

What effect on the current supplying this machine when switching from mains to a genset which generates at 0.8? does this become a perfect power transfer or worsen the voltage-current phase shift and thus current increase?

NB: UK voltages were used for the above numbers, but not that it matters for the answer.

Thanks

When the rating for a generator set is given, the 0.8 you refer to is the rated power factor the machine is designed to supply as the maximum. Therefore, the generator may supply loads with power factors that are 1.00 to a maximum of 0.8. If the generator you are attempting to discuss in your question is rated at 50 kW with a maximum power factor of the load not to exceed 0.8, then your generator exactly matches your load, a motor that operates at 50 kW, 0.8 power factor. Your motor should operate at the same current levels assuming the generator and motor have the same voltage rating at full load.

This does not however cover the affects of motor starting, load variation, etc. on the generator, nor does it discuss the affects of the generator impedance and the circuit impedance supplying the motor. Hope this helps.