If the generator has a decent output voltage meter on it, try applying a reasonably heavy load to the generator, and then switching the load off, whilst watching the voltmeter, to see how much the voltage overshoots. If the overshoot is small, no more than say 10-20% then it might be OK with an inverter type plasma cutter. I had a small Wolf petrol generator that overshot from 230V to more than 300V, when a near maximum load was disconnected. That would not have been good for an inverter setup.