As a rule I would almost double up the power of the genarator, I have exactly the same steempro machine and a 4.5 kva diesel generator.
When you start your motors this is when the generator is working it's hardest so I wouldn't touch anything less than 4000 watts mate, my kipor silent run diesel runs my machine perfect, however it weighs 160 kgs!!!
There are smaller, lighter 4000 w generators out there which are petrol powerered.
Hope this helps
Russ